What to do if your friend likes your girlfriend and she likes him back?

So my girlfriend of almost 2 years has always thought he was the hottest thing ever, but just recently he's admitted to her that he's liked her for a pretty long time. Ever since then they've been hanging out all the time, just the two of them, and staying up until like 2, 3 in the morning talking to each other over IM. For the past couple days she's been trying to make it seem like not that big of a deal, but she's been talking about how sweet he is and all the qualities he has that makes him better then me. Another thing is when I asked her how she can't chose between him and me, her responses are: "I like you two evenly" or "what if I leave you for him and it doesn't work out? Then you won't take me back." Please help me? I don't know what to do anymore...