i don't think this is difficult. it's an overbet and there's only a 1 in 3 chance he wants a call.

You river the mortal nuts. What do you do? Bet something callable/raisable! Opp didn't do this, so opp doesn't have the nuts. You have the next best thing, and should call every time, if only for the size of the pot. That's an $18 bet into an $18 pot. You only have to be right half the time, and I think you're right 4/5 times.