Yeah, I made the mistake of friending a person I have to play later in the tournament, now if we play in the final I'll have to be very careful (though I calculate how many % I have to go before my next match).

He may just be playing you to improve his team though, I find playing Mourinho I never win or get good ratings for my players. This directly effects the attributes given to your players at the end of the game, the better they play, the more they get.

It certainly isn't an obvious option on how to unfriend though.