I can't stand my friends friend?

I've had this good friend for a year now. I really want to hang on to her since I can talk with her about anything without feeling uneasy. I don't have other friends like her. Always when she has had problems I've done my best to make her feel better no matter what. Anyways there's this guy who, I could say, is like a rival to me. Last night we went drinking and he was in the spotlight all night long. He was dancing with her and making her laugh. I hate to admit I'm stunned about how he was able to do that so well. When I tried to talk with him, like I wouldn't care about the situation, he just ignored me. I just can't stand him anymore. The girl talked with him all the time so I focused on my other friends. The next day she mentioned him on the first time we were talking that day and she haven't never really done that. I really fear the guy might take my place. I know this sounds very naive. What do you think I should do?