Players do not have the right to see another player’s mucked cards, even in a called showdown. If collusion is suspected, inform the floorperson.
Note:
Historically, some poker rooms allowed players to see called hands. Players are now allowed to fold their cards, even if another player wants to see the cards. Players were using this loophole to angle shoot each other, annoy other players, and gain useless information from players who are busting out. It’s better for poker if this practice is left in the past. If a player suspects collusion, they should inform the floor.
Sources:
WSOP Live Action Rule #14. A participant may only request to see another participant’s hand if collusion is suspected, and then only in the presence of a floor-person.