Hi OP,
Really shocked and sorry to read your update.
Just to give you an update - I was multiple times the friend they slept with then went back to being friends and they found a proper girlfriend. It didn't make me feel good I can tell you and I never considered that they would feel threatened by me - sure I was the girl that wasn't good enough for a relationship!
Change your perspective on this situation quickly. If she was a cow to you it was probably because she felt threatened by you.
I never behaved like her btw, I'm just telling you how I felt the two or three times it happened.
It was not so awful when I was young and foolish (very early twenties) but I got together with a friend of mine who I had really fallen for in my mid thirties and was gutted when he told me he only wanted to be friends. We did give it a go and it was ok, I was hurt but knew it would pass. Then he mentioned he had a girlfriend and it would be great if we all met up in a group. Knowing him he probably meant it genuinely.
I put on a very brave face, was friendly to him but not too much, very friendly and welcoming to her, spent most of the evening with the others in the group. Inside I was dying. I just wished it was me showing up with my shiny new relationship.
So you don't know how this woman felt. I certainly wouldn't assume she felt sorry for you.