Absolutely, you've invested a huge portion of your life in him and are naturally a little on edge. I think that's why it's so important to nip this in the bud sooner than later, though. If you've felt uncomfortable enough to come on here and ask for advice, why would you carry on like this another MONTH? I mean, maybe another week just to observe a little more interaction but anything past that I think is just letting him know you're okay with things as they stand.

As far as approaching your friend, another commenter has it about right. It sounds like she's rebounding from her breakup and using your fiancé as a self-esteem boost. Unintentionally damaging or not, it's disrespectful. It sounds like your friendship barely even exists outside of your obvious connection to him- and if this is the case, I don't know if the best approach is confronting her directly. It's HIS friendship that has evolved into a level you are unhappy with, so it'd seem more natural for him to be the one that gradually distances himself (rather than you bluntly doing it for him). If you say something it becomes "He won't talk to me because of her" vs. "It seems like he's talking to me less...oh, that's right, he has a fiancée to spend time with..." The former makes you the bad guy and may lead her to attempt more secretive interaction in an effort to maintain the friendship, while the latter leads her to reach the conclusion on her own without making her feel embarrassed. I'm sorry but I personally doubt she is all that unaware. I'd save myself the stress of planning an approach and just ask my fiance to start creating those more reasonable boundaries.