I may be off here but tbh the impression I got from reading your post is that you either have feelings for her or you simply can't deal with the fact that her attention is being diverted away from you towards someone else. And I apologize if my impression is wrong. But since when do people decide of who their friends should date ? While your concerns are valid, why would you make her choose between him and you? That's not fair at all. I understand not wanting to be between her and her boyfriend but that's no reason to end the friendship. Advise her, distance yourself a bit and let her have her own experience even if she ends up learning the hard way.