ifish, I would encourage you to take the huge first difficult
step and move on first. You see, at of this moment you are still in
this endless circle of love like a dogfight including her and her
bf. You can only see yourself and realised how silly things will
really turn out in the future ONLY if you are out of that
circle.
Unlike primatives, we humans have the unique ability to move on and
out of our projected destiny....especially when it becomes endless
or when it's heading towards a certain death....we can make a
choice to step out or step back.
I would like to recommend you to listen to a song by Jay Chou. It's
called Tui Hou (Step Back) and it's in his Still Fantasy
album...track no.5. 
The longer time you spent staying on, the deeper regret you would
be feeling later.
To make it easier for you, don't you used to stay as close/closer
friend towards her because you're worried about her and you somehow
don't feel comfortable/safe seeing her alone? But right now she
already got a bf, so your task is completed and you don't really
have any reasons left to stay on anymore. Instead, you are still
hanging around and searching for another reason to stay on cos
you're kinda attached to this routine mission of yours. I would
encourage you to move on before the feeling of being left behind
consumes you. Trust me, girls move on faster and cleaner than
guys.
And it doesn't means you're leaving or ending the friendship with
her, the friendship between you and her still stays. However, you
know that this is probably a very bad time for you or her to pick
up both your friendships towards each other....it does feels a
little bit wrong, uncomfortable and awkwardness isn't it? You will
know when you are ready to pick up the friendship again with
her...after you've moved on and found someone else to supercede her
in you heart.
Be strong and move on, you'll be stronger everyday unless you chose
to feel weaker everyday.
Good luck.