What's the correct ruling on this situation?

[Brettski]

A couple of points I should probably add to all this.

Firstly, the floor can almost never go wrong by ruling that if there is only one player left with cards, they get the pot. You can apply this principle in a variety of cash game and tournament situations.

Secondly, the OP refers to a cash game, and cash game rules sometimes differ from tournament rules. If this were a tournament and the action was complete, both hands would have been required to be tabled, and both eligible to win the hand at the showdown.

On the other hand in a cash game where ...
a) a flop is dealt,
b) a player moves all in and is called, and
c) a player chooses to muck their hand before the rest of the board is dealt out,
... then I would be perfectly happy ruling that the pot must be awarded to the only player remaining with cards.