It's a big no-no to sleep on the floor. Several reasons: Turbulence, the oxygen level is far lower down at floor level, you could get stepped/stood on, and so on. I'm quite shocked that the crew didn't see this chap, was he hidden under piles of blankets? If that was the case, he would be more difficult to spot, but the crew should have been more observant. You could have said something, of course, saying you were concerned about him being unwell, but you shouldn't have to iyswim.
Incidentally, I find babies and small children asleep on the floor quite often. When I tell the parents they can't sleep there, including the reasons why, they still get annoyed. Some parents even put their baby straight on a bassinett table! I don't think I'll ever get used to some of the weird things some customers do.