You talk to them and tell them how that's metagaming.

Then explain that if they're going to do things like this you're not going to let them join in for stuff like this.

I had a player do that during a game of liar's dice (IC). Only one of our characters actually made it to the table (via choice, the other players acted as look outs and back up if he needed it) so the other players played NPC opponents. One player purposefully lost so the rogue would have a better chance at winning.

Not a good day.