Whenever something bad happens to us we face a choice, whether we realize it or not: We can either let it defeat us–or with God's help, we can defeat it.
Unfortunately, apparently your nephew has let this tragedy defeat him, by allowing bitterness and anger to take root in his mind and turn him into a very unhappy person. Bitterness is like an acid that eats away at our souls, and no good ever comes from it–either for us or for those around us.
Don't misunderstand me, however; no doubt what he has gone through would be hard on any of us. Even Job complained in the midst of his suffering, "I loathe my very life; therefore I will give free rein to my complaint and speak out in the bitterness of my soul" (Job 10:1). But listen: Job didn't stay that way! Instead, he turned his life over to God and committed the future into His hands–even if he didn't understand why he was suffering so much. The same can happen to your nephew.
Pray for him, and encourage him to open his heart and life to Jesus Christ. Christ loves him so much that He was willing to endure the suffering of the cross so your nephew could be saved. Christ gives us hope, even when we don't understand why some things happen to us. With Christ we can have "the peace of God, which transcends all understanding" (Philippians 4:7).