I was in your shoes a few times.
Since 2005, I've had 3 girlfriends:
Girlfriend #1 - Two year relationship, she wanted to get married and I said no. She cheated on me, I found out.
Girlfriend #2 - Girlfriend #1's best friend...

We dated for a month and still talk once in a blue moon.
Girlfriend #3 - Left #2 to get with this girl, #2 was kinda psycho. Little did I know, this girl was even more psycho... but in the sack. So much that our entire 8 month relationship was made up of sex, sex, sex, sex, and movies. I went out of town for a weekend and she ended up getting back with her ex... we continued to have sex. For a year and a half while she dated him. We broke up in summer of 2008 - I'm just now fully getting over her, a year after our last encounter.
I still talk to #2 every once in a while, but it's usually her trying to rub it in that she's getting married. She even asked me to photograph her wedding... and for a large discount.
With my advice, I'd say bone her friend (or 4) and forget about her and her friend(s). IMO...