since she knows your girlfriend very well and knows you are in a serious relationship ... i think what she needs is obvious...  some time to enjoy what she has been playing with since high school .. its left with you to give it to her how she wants it or walk away its simply a f**k or a brake ... you can chop her and tell her it cant last between you... she gets annoyed and brake away or you say no rudely and lose her friendship :'( ...or play the good friend and mess up you relationship when she becomes obsessed about you...