An atheist gave me a few questions/arguments. He asked something on the lines of “Why would God kill babies and the unborn within the flood which had done no wrong”.
To a materialist – for whom this life on earth is the only life that exists, such that “loss of life” is an absolute, and which is the greatest evil possible – every death is an injustice; therefore, for materialists (among whom are many atheists, although I don’t know if your friend is a materialist), every death is a demonstration of the way that God is ‘evil’ in His treatment of us.
For a Christian, though, this life is transitory; moreover, our focus in this life includes attainment of eternal life in heaven. Therefore, the death of any person – while it is sad and a cause of grief – is not the ultimate evil; rather, the ultimate evil would be eternal separation from God.
If we want to deal with the Noah story literally and on its face – which is what he seems to want to do – then we see that evil had overcome the earth: “the LORD saw how great the wickedness of human beings was on earth, and how every desire that their heart conceived was always nothing but evil” (Gen 6:5). I think that a reasonable person would conclude that, if wicked parents were to raise their “innocent children” in that wicked world, then those children would be corrupted in that experience. Therefore, (again, if we’re treating this strictly literally), I think that one could make the case that, by not allowing them to grow up and be corrupted, God is being
merciful to the ‘innocents’, by “bringing them home” to Himself in heaven.
Another thing he seemed confused about is that why did God do a bet with the devil and allow Job to suffer (this is what he claimed, I’m not 100% sure what it shows within the book of Job).
Again, there’s the question about whether, and to what extent, we take the Job story as literal (as opposed to acting as a parable). Regardless, though, we’ve got the same materialist/Christian dichotomy in play here. If a materialist sees this life as all he has, then any suffering he experiences here is an injustice. To a Christian, though, suffering can be redemptive – it can lead us to recognize eternal truths and point us in the direction of our eternal destiny; suffering can be a
good thing, both for us who suffer and those who witness suffering that is borne heroically and faithfully.
So, your friend needs to actually read the Book of Job.

The entire arc of the story is that Job bears his suffering heroically, defying his family and friends who ask him to give in to the pain, to despair and therefore declare himself ‘cursed.’ Job refuses, and not only does he receive his “eternal reward” (as the people of that day would have understood it), but his witness to faithfulness in the face of trial is an object lesson to all who have heard the story. “Why does God allow Job to suffer,” then? Because, through that suffering, Job was redeemed and we are edified.
The other question, of course, is “why a bet with the devil?” I would assert that it’s a device within the narrative in order to frame up the action of the story. It’s the prologue, and it helps us understand what’s going on and what will happen. It acclimates us to the story: it’s not about
what happens to Job, so to speak; it’s about the bigger story – that no amount of suffering implies that the sufferer is unjust or that God has condemned them.