I’ve been where you are. In two different ways. One time a girl I feel in love with had a boyfriend. I told her I was in love with her and she cheated on her boyfriend with me. It was fun in the moment but ultimately I lost her. 2nd time I fell in love with my best friend that has a boyfriend. If I tell her things will get weird I think so I’ve kept it to myself this whole time. It takes a lot of me mentally. So I’ve been in both camps. What would I recommend? Honestly I don’t know. I’ve lost both times. If I was you I think I would ask more details about this guy she likes that isn’t her boyfriend. You could always go the route if becoming her best friend. Romantic relationships always work better with a strong foundation, remember that. So TL:DR, there isn’t a perfect answer. Either way you’re in pain. If I was you I might risk it, but you aren’t me. You’re you, do what you know is best. Whatever that might be.