Really sorry to hear about that, Feanor. Such stuff seems to happen to nice guys only. I think you have managed it very well so far.
However...
If she puts herself in a position where he is ever again, it's over, over, over. And I would tell her that, flat out. If she has to ditch some friends to make that happen, so be it.
...our resident conservative whacko is right (sorry, jlutin
).
That was an awkward situation to begin with. Seems to me at least one of her friends don't like you much, and wants to cause some trouble here.