Be prepared to sit there feeling about two inches tall. You'd better be ready to say a quick prayer and say, "Come Holy Spirit!" and not try to respond on your own lest you give a pretty sound theologically correct but not a very pastoral statement such as "well, you know, God wouldn't put you through anything he wouldn't put himself through---after all, He died for you and for me on the cross." For someone who is hating God right then, the thought of Jesus dying 2000 years ago on the cross is too abstract. And if that doesn't get you cussed out saying something like, "Well, God never gives us more than we can handle" certainly will. Say that and you will probably get you chewed out royally. "Are you kidding me? Why don't you try walking in my shoes, having to have someone help you go to the bathroom! I've suffered so much. Why can't he just give me happiness for awhile! I'm a good person. I've never hurt anyone. I don't deserve this."
What do you say to someone who has been hurt so much by God that they are hating the Lord or that they have lost their faith. Well, if you are smart, you aren't going to say anything. Hopefully you shut up, pray, and let the Lord do the talking through you. There is nothing you can say that will make them feel better or bring them closer to God. The only thing that can help them is God's grace. There is a good chance that this person has a long road to ho before they find their way back to God and just like you can't convince one person to forgive another one, you won't be able to convince this person to forgive God. They have to allow themselves to be infused with God's grace and do it on their own.
But, if you are like me, and just have to try to fix everything because it's your nature as a cop then be simple. First, invoke the Holy Spirit and ask the Lord to guide you. And from there . . . I don't know. I mean, I understand that God's ways aren't our own and that we sometimes can't understand His will. I understand that. But when you aren't the one who is hurting you feel helpless. All you can do is pray, offer some sort of mortification in order to relieve some of their pain, so that they find it easier to turn to the Lord.
Prayer, prayer is the answer. If they won't do it then pray for them.