Personally, I would have been pretty hurt/pissed as well. Regardless of the relationship or situation, it would be naive to think (deep down) that your partner never thinks about being with someone else. It's just human nature! That said, just like certain things need to be kept on the DL, this is no exception. I don't broadcast to my wife my Fav 5... she'd be a fool to think I don't have one but it would be mean nonetheless. Obviously the same applies the other way around. Does she think about other dudes? As much as I hate to think it, logic suggests she must. My macho side says this should only be a 1 way street but my logical side says that wouldn't be fair. We both think about others from time to time... but that's where it stops. If we were to verbalize our thoughts to the other (knowing full well how much it would hurt/offend them -cuz how could it not?) we'd clearly be selfish types and not give a damn about the others emotions.
And there in lies the problem here...
Unless she's a few screws short of an orgy (which... lol) she HAD to of known this would piss you off. I mean... a friend of yours no less!! We're not even talking about some celebrity that she'd have less than 0 chance with in real life... a F'in friend!!! Big WTF to that partner. Don't listen to what some of these posters are saying... you have every right to be PO'ed!!
But before you do anything rash... I'd suggest looking within and determining whether or not you had this coming. What do I mean by that? There's a chance she has 0 interest in your friend. There's a chance she has 0 interest in anyone and is madly inlove with you. So why say what she said? Simple really. Women love to play games. So do we... but we're merely amateurs in comparison. Have you been neglecting her? Not from your vantage but from hers. Women (generally) need a lot more attention/affection/etc than we do... and certainly more than we often think they need/want. So if you think you may have fallen short in this (or a related) area lately... perhaps this was her way of trying to light a fire under your ass.
Or maybe she's just a *****... lol
Sorry, that last line was a joke... sort've. Good luck
While there was a stage in my life (albeit a more egotistical, less mature one) where I'd probably get ripped just thinking about my girl THINKING about someone else... I've mellowed a bit since.