Most Helpful Girl
Hard situation. You say she has a golden personality, but form experience unless you live with her on a day to day basis you will never really know her true personality, only the good side of her.
If your friend is treating her with respect and honesty etc and they seem good together I would leave it alone. If he's a good friend you need to let go of her and move on. Maybe stop hanging out with her at parties so much and concentrate on other people there. You can never really move on until you can forget about her. When you meet other potential partners, you will never meet anyone good if you keep comparing this girl with them.
Remember they keep breaking up and getting back together, she can't be that perfect personality wise if there always doing this. You also said she flirts with you. She may seem amazing but any girl that flirts with her boyfriends friends has issues , trust me on this. No girl who loved there boyfriend or treated him with respect would do this.
I think your more obsessed with the idea of her then her and her being beautiful tends to amplify the situation. This happens a lot by the way, people love the idea of the person but then they get them and the idea isn't as great as the person in the end.
I know it's a hard situation but If you are honestly meant to be with each other, it will happen. But don't betray your good friends for it. You could always travel and have no contact for a few months, this might help.