Since most people r saying go back and bang her, I'll try some real advice. Being as young as u guys r, her referring to herself as lesbian may not be true, she's probably more bi, this isn't true for everybody but at thst age u really haven't had enough experiences yet to know what you really like. I'm sure she feels safe with you, as you've been close friends for so long and I assume you never made any advances toward her, and that feeling of safe may have begun a feeling of attraction. You guys need to talk, but if you're ok with an open relationship I would say go for it. Some of the best relationships I've been in were when we started as friends, most I'm still friends with after the break up, as long as it wasn't working out mutually, one stalked me and I had to get a restaining order, but I digress...it sounds like you really care about her or you wouldn't be so upset. If ur not ok with the open then maybe if they break up you can see if she still wants to explore it, if ur into it of course. Anyway you said in an edit u were going back to talk, how'd it go?