You aren't forced to show in a cash game at showdown to take the pot. I've seen times where a player first to act on river bluffs, gets called, then mucks. As the only live player left, the caller takes the pot without having to show.
Then you’ve seen it done horribly wrong. First, the player first to act can’t muck his hand. Only the dealer does that. He discarded his hand and as dealer, I’m not mucking that hand at showdown until the other player tables his hand. If the other player refuses to table two cards after being warned he/she will not be awarded pot without doing so, the dealer should call over whoever is in charge. At that point, the initial player will probably be allowed to table his/her hand for the pot. (Note: Dealer should never expose either hand under any circumstances unless directed to do so by the person in charge.)