Okay, so for example if my opponent for that week has Brady as his QB then he benefits from Brady having a great day. I may well have Gronk, then I benefit too but only when Gronk catches a pass.
But because you have 10 players, almost every game will have some sort of rooting conflict of interest. I can honestly say that I have never ever rooted for something bad to happen to the Pats in order to win my fantasy match. Not that it matters one way or the other. Given your friends' choice, I would say he has only demonstrated that he wasn't really a true Pats fan, there's no way I would take $125 over the Pats winning a Super Bowl.
But it raises an interesting question - how much would it take for you to switch your allegiance? I'm thinking at least 10K, and if I were financially better off then we'd start adding zeroes to it. What would be your price?