Should I tell her?

My two friends, mary and anthony are going out. Before they started going out me and anthony were talking and he told me that he liked mary but not enough to go out with her. He just thought she was hot. Me and mary are really close and tell each other everything. I didnt tell her about this because I didnt think they would end up going out. Later on that day anthony asked her out. I didnt think to much about what he told me. They have been going out for almost two months now, but anthony has been acting like a jerk and ignoring her. He has been flirting with one of our other friends. I told my boyfriend about what anthony told me. He said I have to tell mary, but I dont know if I should. I know she’s hurting but there are so many things stopping me.

1. I will lose the trust of mary
2. I will lose anthony as a friend
3. I’m scared to make things worse

I dont know what to do…should I tell her or not???