It's a tough one isn't it. And I'm sorry/happy to say that it isn't unusual in the slightest. Often, when we are in a close relationship with someone, even if that relationship is entirely platonic, we do have some problems in separating our sexual and physical feelings with the emotional link that we have with these people. Occasionally, this leads to a special relationship, but in the majority of cases, it leads to a breakdown in the relationship, and I presume that that isn't what you are after. I'm guessing- having been in the situation before myself- that you want, on occasion, to blurt it all out, and tell her. Don't. For your own sake, don't.
Normally, I'd tell you to be true to yourself, but the fact that swung it for me is her long term relationship. The fact that she is in it, and hasn't told you that she is unhappy in it, means that she must have pretty strong feelings for him.
As her friend, I don't think you should interrupt what they have. Not just yet, anyway.
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'Love is, like, really important. It's good to be loved.
Even if it's only for one night.'