I'm about to win this argument.
Both of the groups are right. It's multiplayer if it was intended to be multiplayer and if it is played as multiplayer despite intention. So it doesn't matter if no-one is playing it multiplayer, as long as it was intended to be multiplayer it is multiplayer. And it doesn't matter if it was originally meant to be multiplayer, as long as it is played with several players (while not specified, several is defined as more than 2 but not many) it is multiplayer. If it is played by 2 players at the same time, it is specifically called a 2-player game. As Wikipedia says, "A two-player game is a multiplayer game that is played by precisely two players. This is distinct from a solitaire game, which is played by only one player
." Therefore, both of you are right on the points of intent vs actuality, unless it involves only 2 players at once in which case you were both wrong, and it is instead a variant of multiplayer.