What would you do?

I think that in this particular situation it would be very hard to prove that the student didn't actually forget to write his answer on the page. Unless there were no written calculations, say, for an IV drip rate. It may have looked to you like he cheated, but it's hard to say for sure. It seems like the real problem was with collecting the tests at the end, and how students were given access to other people's answers before turning their tests in and a different way of collecting at the end should be used. Perhaps you could mention it to your professors in that light instead?

What you saw was very upsetting I'm sure. It isn't fair to study hard and then see someone cheat their way through. But it will really come down to your word against his. If the teacher didn't see it then how are they supposed to decide who is telling the truth? You have no proof and he will simply deny it.

I would drop it because one time I really forgot to put the answer to the math questions on the back of my scantron and when I went to hand the test in I remembered so if someone said I was cheating that would have ruined my life over someone not minding their business worry about yourself as long as you do the right thing there is nothing to worry about

I would let it go unless it was a really big deal, like the final exam or multiple questions on one test. Maybe you just thought he looked at someone else's test, you never really know. It's not worth it.

I don't know what the others have said thus far, but I'd hold my tongue. This is hard to prove and the proverbial "poop" could hit the fan. Try not to get caught up in the drama. Sometimes the fight just ain't worth it in the end.