I've been 'the other guy' in this story. My girlfriend and I have been together for four years and have a 1 year old son. She's in uni and twice has she had guys befriend her and then a few months later express their romantic interests. The one who came along after the birth of our son did not appear to care that his interest could put our family unity in jeopardy and severely hurt her chances of being able to finish university (a child and two students requires careful time management which is more or less impossible to accomplish alone).
I would give you this advice:
If the girl is your friend, then you have already been 'assessed' as boyfriend material. Even those in the strongest of relationships may imagine themselves in better situations with more desirable partners, and if you are close then she will undoubtedly have considered you in this way. She is, however, still in a relationship and you must assume that she has made the decision to continue that relationship because she genuinely wishes to be in it.
Assuming anything else, such as that she doesn't know what she's doing, or that the other guy is exerting undue influence on her, has two effects. Firstly, it perfectly justifies intervention because her relationship is 'not really real' (you can see the truth, but she cannot). Secondly, and this is the part you will find difficult to reconcile, it also means that she is incapable of making informed decisions about her own life, and therefore as liable to make a bad decision by choosing you as she is by sticking with her boyfriend.
You can see how allowing her to make her own decisions is the morally superior option here. By assuming that this girl is bright enough to make up her own mind, you not only elevate her status to one which is superior to a domesticated animal (a good thing, I hope you agree), but you also save her and her boyfriend a lot of heartache. If she returns the interest, then you will become 'the other guy' and the chances of your relationship getting off to a smooth start will be somewhere in the region of impossible. If she says no, then your friendship will become incredibly awkward and you will probably drift apart in the following weeks and months.
The conclusion is this: If you want to have any chance with her, and if you want to respect the decisions she has made in her life, then leave it alone. Stay friends with her and, at most, ask her how her relationship with her boyfriend is. If you're not confident that she has ever thought of you in that way, asking her is likely to get her to do so. So will telling her about the part of your romantic life which does not involve her (e.g by telling her that you're waiting to meet the right person). You must assume that any sensible girl will make the correct decisions about her future - any other conclusion means that your love interest is, in your view, incapable of making her own decisions. Treat her with respect and don't fall into this trap. Date other girls and see if your interest in her is still there. If it is, bide your time and if things go wrong for them, then tell her how you have felt and you may discover that part of the reason things didn't work out was that she had her mind on someone else.