Pretty rough. That's going to be hard not to call because it seems so much more likely that he just has a jack, not KJ. I don't think it's a terrible call. I can see though why he called the flop bet - with three callers ahead of him he's getting very good pot odds on his overcards. And if he gets a 9 or a Q on the turn, he's picked up a good draw - so it's like 8 outs to make a good 8 out draw, and 6 outs to make top pair decent kicker. He can't absolutely know where he stands in the hand, but with pot odds that good, why let it go?