Friend in toxic relationship

One of my best friends is bi and she’s dating another girl, who is also a friend of mine. She’s a bad influence on my best friend though and their relationship quickly became toxic, but they don’t see that. I love them individually, they’re just not good for each other at all. I want to tell my best friend but I’m afraid she’ll think I’m against it it’s because she’s not straight (that’s not the problem here) or she’ll become protective and angry. I don’t want to hurt her either. It’s also taking an awful emotional toll on my other best friend. Our group of friends agree that it’s toxic. How do we help while still showing her we love her and aren’t against her. We all really care about her.