I prefer asking them to prove that P != NP or P == NP. Someday a candidate will answer it, I'll steal their answer and be famous!
On a more serious note, though, I think it's completely fair. Most NP complete problems are easy to solve, they just run very slowly. Unless the job requires them to know a lot about complexity theory, though, all they need to demonstrate is that they understand the solution will be slow. Bonus points if they know it's non-polynomial time, gold star if they know it's NP complete.
