happens a lot in games, have seen people friends join a match get placed on the opposing team then quit out to give their mates an advantage, you see this sort of thing all over the net in games unless some form of penalty is inflicted against dirty players, which it normally isn't.
As with many games, even when partied up something can go wrong and a friend will be put on the other team, they'll also use it to help their pals.
If I had a a dollar for every idiot that ruined a game I was playing I'd be a rich man. Some do it out of boredom, some do it because they are just arseholes, or others are just bad sports. Nothing says fun like picking up a sniper rifle in a game and having a team mate try and kill you because they wanted it, not caring if they get booted..... in games that allow it.
But this sort of things happens in real life so it's bound to spill over into games, and is getting worse thanks to kids being spoiled rotten these days and being the lazy don't care 4chan /b/ troll generation.