[Tilt]

There are two possibilities where he takes this line. One is that he he went for the c/r on the flop with a set, but when the turn made a straight he also picked up a flush draw so he bet anyway.

The other is that he he has a red straight, is afraid of the freeroll on the turn, but then figures that paired board didn't hit you and bets it out.

I think its the second situation most of the time. Set + flush draw is just a lot rarer probabilistically speaking, plus the chances that he didn't just bet out on the flop with a set + backdoor flush draw makes that scenario even less likely. I don't think he has two pair + straight, as he likely would have raised that combo preflop or reraised you on the turn.

I think he is betting the scare card here ~70% of the time, I just don't know whether thats often enough to call. Calling has some metagame benefits however, so I am leaning toward that.