I would say to him that's no reason to be an atheist. Most suffering in our lives is caused by other people, not God.
God has to allow us to do good so he can bless and reward us. He can't be arbitrary about these things; he is as bound by eternal laws as we are by natural laws. He has to let us do good.
The problem is that the ability to do good is also the ability to do evil. God can't stop evil without also stopping good, and that would destroy his plan for our happiness and salvation. God has to let us do evil, if that is what we insist on doing. He can normally only step in when entire societies become irredeemably corrupt.
Besides, we would all be very sorry indeed if justice were immediate or proactive.
Also I would encourage the atheist to do what millions do every year -- to know God for himself through his own experiences with him. I'd tell him not to bother to move the goalposts, because it's more evidence than he has to shore up his atheism. I would also urge him to persist in his efforts until some day he feels God's love for him. Then he will know for himself that God couldn't cause all the suffering that is in life. He doesn't have some master plan that specifies everything we shall ever say, do, or think. He has to respect our agency -- up to a point, and we're rapidly approaching that point.