Oooooh, this is very interesting, I really want to hear what people have to say on this topic now. Because as it happens, I have a story planned out that I'll soon be writing where one of the main characters is in this exact scenario. It's made slightly more complicated, however, because she's in a band with her love interest, who is her closest friend, said love interest is dating a girl who had a really bad childhood involving sexual abuse by her uncle, and the main girl in this situation is also a closeted bisexual who ends up in a relationship with a close female friend (who she also loves, but not as much as she loves her main male crush/bandmate/best friend). So, like the OP's scenario, but gender-flipped, and more twisted and complex.
Anyway, I was in a situation similar to this in my first year of university. I fancied one of my female flatmates, but left it too long to make a move, and she ended up in a sex-buddy thing with another of the guys in our flat. A few months down the line and they were dating properly, and are still a couple now. Though I've got over it in the past couple of years, so it doesn't make any difference to me now, and I wish them both all the best for the future. But at the time, it did hurt a little, and I did regret not making a move sooner...