@Kyle-S
Hey, first of all you are not a bad person. I was in a similar situation with my best friend and from my experience I think you should talk it out with her, tell her how you feel because maybe she feels the same but is afraid to tell you.
You can even tell her if you think the guy is bad for her or if he's changing her but if she won't listen to you you'll know that at least you tried to help her because you care for her.