question about showing your cards when villain folds and there is other player all in

whats the ruling here?

cash game, 3 player goes to a flop and one of them is all in.

player A is all in, on the flop player B bets and player C calls so there is a sidepot.
turn goes check/check
on the river player B bets and C folds.
player A show his cards and they are better than player Bs holding.

Does B has to show his cards in order to get the sidepot? or since C already folded he can get it without showing?