I think you are feeling a but left out. At the start of a new relationship, alot of people tend to 'forget' their friends and give all their time and energy to their new partner.
It is not your place to start gossiping about what you have heard and telling her to dump him, to be honest at her age it will probably have the opposite effect and make her stronger with him and more likely to dump you. She will just think you are stirring up trouble.
I know you are concerned but all you can do it wait it out and be there for her when it comes to an end - which it will, hardly anybody stays with the fella they met when they were 14!!