Interesting, i thought this situation is a bit more subtle.
The point is, in the bank situation, the opponent is just making, sort of a, syntax error if we make analogy to programming. I will fully expect in his mind the bank should make the money when he plays it right.
On the other hand, in this kind of game, he may have built his deck in the hope of getting the VP tokens from possession. At the point when his deck is finally clicking, sometimes it can be a hard thing to swallow that his initial assumption was wrong.
That being said, I think I will tell my opponent that he will not get the points if the game is still close; if not, I'll probably try to finish as soon as possible and point it out afterwards.
I think, conceptually I'll avoid giving out strategical recommendations unless my opponent asks me about it (this is also the situation I think which you can accidentally sound presumptuous and make your opponent unhappy) but will correct the technical issues.
(Admittedly there are not that many technical issues in Dominion, Bank, Outpost, Tactician, Possession are about the only ones that I can think of right now.)