Must B make an aggressive bet when last to act after A exposes his hand?

[Steven]

This sequence happened recently at a table I was playing at.
2 players left in hand.
After the river card, Player A bets 2000,
Player B raises to 4000
Player A then tosses out another 4000 with no verbal and turns over his hand, apparently thinking he had just called.
Since it was a raise, Player B acted and tossed in the extra 2000 and claimed the better hand.

A discussion arose as to whether Player B MUST re-raise the hand since he can see he has the better hand!
Would you consider the call to be soft play, similar to checking or calling a nut hand when last to act?

Or could you claim that Player A might be promoting a chip dump by exposing his hand and "inviting"  Player B to re-raise if he has a better hand?

Btw, the actual sequence was a quite benign situation in which Player A clearly made a mistake and Player B was just trying to oblige!

[chet]

Steven:

I do not think that Player B should be forced to raise, unless he has the absolute NUT hand.  Why should B take advantage of A's error?

Maybe B is just a nice guy who realized that A, an inexperienced player, made and error and did not want to loose him from the game.  I can come up with numerous reasons to not force a raise in the example you gave, again assuming B does not have the absolute NUT hand.

Chet

[Tristan]

I agree with Chet, I don't see any reason that B should be forced to raise unless they had the nut hand.