This is a suggestion that is adding a rule to something that is already against the rules. Secret Diplomacy.
People could be in cahoots in a variety of ways, sure. One is pm's. One is emails. One is skype or other voice chat. One is phone. Another is, as this suggestion intimates, "playing with a rl friend where you can either speak with them about what is happening or watch each other as it's happening."
These are all still banned under the ban on "secret diplomacy."
So, no new rule is needed.
Now, if the concern is, "because player 1 likes player z so does things to help z win if player 1 cannot win," well, as someone suggested, clanmates, chatmates, ranks... all sorts of things affect whether you will "favor" one player over another, too many to ban unless there is some sort of global limit on how many standard games player 1 and player z can participate in together.
So, maybe a global limit for any 2 players on standard public games (or terminator, or assassin) would address all situations, rather than adding a rule where an existing rule already covers the problem.