Don't expect a woman to give you a straight-up answer on this one. It's tough to get women to admit to resenting one another. They're always trying to out-dress each other. Some are obsessed with flaunting designer names. Others are shy, lack self-confidence, or feel intimidated by attractive, confident women. And we all know the ones that need to be the focus, and get pissed when some other cutie steals some attention away. I have no doubt that the scenario you describe has some truth to it. I wouldn't say it happens every time a group of pretty girls are friends. But I'm sure it happens enough times to give merit to what you're buddy says. Consider the fact that all men are pigs, you can see how a man dating a girl with hot friends might do very little to put an end to any flirtatious behavior he might encounter. My money says he might even encourage it.
I think women take more of a conscience hit than men do. That's not to say that men don't feel a hit too, but I think the genders differ in how they file away their indiscretions. In any case, the underlying motivations for affairs of this nature aren't easily admitted to, and may not even be understood or realized. So I can't see a woman saying "Yeah, this happens cuz I did it."
I'm a college age woman and I'll admit to this since it's anonymous :). It's totally true. I am pretty and have pretty friends and one in particular is just flat out beautiful and yes she's my good friend but there is some deep hidden resentment over her looks. And yes, I have fucked two of her boyfriends