I hope you can use some of this in answer to your question. The question is actually the "Why" behind our whole existence and is one that most of us have probably asked at some point in our lives. I know I certainly struggled with it and was surprised when out of the blue, God granted me the revelation to understand the answer one day. This came by direct revelation first, then confirmed in the scripture by the Holy Spirit, so once you see it, I think it will help you. Even though it is difficult to understand, there is great wisdom in what God has done, and wisdom is justified of her children (Luke 7:35). Get her to see the wisdom in this creation and our existence. The blacksmith didn't create fire, but he definitely puts it to good use!
Let's first set some ground work:
God is love.
I John 4:8 He that loveth not knoweth not God; for God is love.
I John 4:16 And we have known and believed the love that God hath to us. God is love; and he that dwelleth in love dwelleth in God, and God in him.
As we will see, everything that God does is motivated by and reveals his love. Since this is his very nature, we need to see how his love is revealed in his response to Lucifer's rebellion and our creation.
God must be justified in all that he does or otherwise he becomes unrighteous.
Gen 18:25 That be far from thee to do after this manner, to slay the righteous with the wicked: and that the righteous should be as the wicked, that be far from thee: Shall not the Judge of all the earth do right?
Rom. 3:4 God forbid: yea, let God be true, but every man a liar; as it is written, That thou mightest be justified in thy sayings, and mightest overcome when thou art judged.
Rom. 3:26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus. (also Rom. 2:5-6)
I had never thought about God having to be justified in what he does until reading Romans 3:26 one day and realizing that God, himself, must be justified (just) in justifying those that believe. (On a side note, Abraham being willing to offer his only son for God, justified God in offering his only begotten son for man.)
Where there is no law, there is no transgression.
Rom. 4:15 Because the law worketh wrath: for where no law is, there is no transgression.
This is an important point, because a judge cannot sentence a person for doing something when there is no law against doing it. As parents, we punish our children for doing things wrong, sometimes when the child doesn't know it's wrong, and we are being unjust parents when we do. Well, God doesn't do that. His commandments, the law, must be made clear and published to all before someone can be declared guilty as a transgressor and be punished. Because rebellion and iniquity were not formed in the mind of God, but found in the heart of Lucifer (Eze. 28:15 Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.), God had not created any commandments against rebellion prior to it being formed. Legally, Lucifer couldn't be punished for what he did. This came as a surprise to me, but the Holy Spirit spoke this to me and granted me the revelation to this question as I walked to my car after work one day.
Why didn't God just get rid of Satan and spare mankind? Because it would be unjust of him to do so! I realize it is difficult for someone who is in jail for being unjust their self, to understand the principle of justification, but God must be justified in everything he does or he is not worthy of our worship. Does she want an unjust God?
Everyone that declares himself righteous must be able to pass judgment or he is a hypocrite. Whether you realize it or not, we are all guilty of judging God continually. Certainly whatever God did in response to Lucifer's rebellion, must pass the judgment of all his created beings and reveal the love of God and the wisdom of God because those attributes are part of God's very nature. Many people think, just because he is God, that he can do whatever he imagines. Well, yes and no! He can't do evil, because he doesn't imagine himself doing evil. He couldn't just destroy Lucifer and the ones that followed him, because he doesn't desire destruction, he desires redemption and restoration. If he had destroyed them, he would have ruled by fear for eternity, just like any other tyrant who takes the law into his own hands. See how God couldn't take this course of action.
Why didn't God spare mankind the ravages of the devil and the destructiveness of sin?
God cannot create character, it must be formed in the fire of adversity. If God could have created character, then Lucifer would have been an example of it, because he was one of his highest creations. We see this same example all the time with children "born with a silver spoon in their mouths" going bad. The trials and tribulations of this life are not the enemy that they appear on the surface, but are our opportunities to develop character. The only real tragedy in life, is for someone to die lost, not the rapes, robberies, cruelties, and murders. With great adversity comes great opportunity to grow.
I Tim. 2:14 And Adam was not deceived, but the woman being deceived was in the transgression.
We had no choice of being born into this world (into the first Adam), but we have a choice to be born again (into the second Adam, Jesus Christ) Eve transgressed God's commandment because she was deceived, but Adam chose to do so and sacrifice his relationship with God, to stand with his mate. Likewise, Christ, the second Adam, chose to sacrifice himself to redeem us, so we can stand in him and fully trust him.
When Lucifer rebelled along with a third of the angels, it destroyed the harmony in heaven, God had to wisely and righteously deal with rebellion on an eternal basis (once for all), restore the harmony in heaven, and replace the fallen angels with a higher creation. The whole purpose of our current existence is to address these issues. God chose to create a higher race of beings in his image who would become not servants, but family. Each being would be tested before they were given a home in heaven. Before man was created, hell was created and a law was given to the angels specifying what transgressions would send them to hell (Mat. 25:41). Hell was not prepared for man, but all that reject God's salvation will have no other place to go. God purposely placed man within Satan's territory, knowing that man would be deceived into rebellion against God. Knowing this, he also created a plan of eternal redemption for man. Since man would require a redeemer, Christ was already ordained to redeem mankind before man was ever created, I Peter 1:18-20. Christ who would learn obedience through suffering, was also appointed to be the judge of the whole creation. Who then could question the righteous judgment of one who had become the suffering servant of all. Satan's eternal judgment would then be sealed, not because of his original rebellion against God, but because of all his transgressions against mankind, crucifying a righteous man being his greatest.
Obviously, we aren't told all of God's dealings with Lucifer and the fallen angels. Exactly what law God created for them and any plan of redemption offered to them is not specified in the scripture. However, given God's nature, it is without question to me that God has extended as much grace and mercy to those rebels that he is justified in doing.
James 2:12-13 So speak ye, and so do, as they that shall be judged by the law of liberty. For he shall have judgment without mercy, that hath showed no mercy; and mercy rejoiceth against judgment.
God is a merciful God, and in all that's happened and all that's going on, his mercy, wisdom, and love are being revealed, not only to mankind but to the angels in heaven.
The simple answer to her question is that: God is righteous and must be justified in everything that he does. God is also love and must show love and mercy even to Satan. We are actually the benefactors of Lucifer's fall, and not the victims. She can choose whether to see herself as a victim of all of the chaos that Satan's rampage has caused, or be victorious and am overcomer through Christ Jesus. Romans 8:36-37 says it clearly: As it is written, For thy sake we are killed all the day long; we are accounted as sheep for the slaughter. Nay, in all these things we are more than conquerors through him that loved us. Yes, God's love is revealed in this chaos by giving us the opportunity to develop something that even God, himself, couldn't create: character. His wisdom is displayed every way we look at it!