What's the rule?

So I've only ever played online.  I had a game today where the only legal move for my opponent was en passant but they apparently didn't know the rule.  They looked at the position for a few minutes and eventually resigned.

What happens in over the board games if opponent can't find a legal move even if there is one?  Do they just time out?

That's an interesting question haha I've been playing OTB for more than 12 years and I've never encountered a situation where an opponent can't find a legal move. Also it sounds extremely rare for an en-passant to be the only legal move for a player!

In a practical scenario, if your opponent can't find a single legal move over the board, they would probably think that the position is a stalemate and therefore a draw. They'll probably then try to bring an arbiter over to make this claim, but once the arbiter sees that there are still legal move(s) to be played they'll just be asked to play on and try to find it. If the player really can't, I guess it is really game over XD

Not that it matters for the question, but opponents king was in check and only escape was en passant so I don't think they'd call stalemate.