What should I do about my kemonomimi friend?? /srs

Hi, this is pretty serious, I guess. if you count boundary breaking as very serious.

I got into kemonomimi stuff from my friend, we'll call her A. A always talked about her discord boyfriend(s) whilst still claiming to be lesbian. She's always been an kemonomimi and is very active about it. I never suspected she would use it for anything else than cosplay and outfits. She often wears dog tails and such like that.

Recently, she started officially dating one of her boyfriends, we'll call her W. It's important to note that A is 13 and claiming to be 19, whilst W is 22. At first, A would talk to me about nice things W would do. (Such as giving her money for doordash, complimenting her, encouraging her to eat, etc.) She would let me look at some of the chats and, if I ever requested, let me look at images of him. One of the things I've noticed is that W always called her things such as "Dumb puppy" or other praising things that always included puppy or doggy. Idk if this is only a nickname or something more pet-play like.

About a week ago, when me and A were alone, she started talking about more personal things they'd talked about. Personal as in intimate things that friends shouldn't know about. I, as kindly as I could, asked her to not talk to me about those sorts of things as it makes me largely uncomfortable. She said it was okay because W's her boyfriend and she trusts me with things like this. She also claimed I was overreacting, and I was being "Disrespectful to her boyfriend as always." The only disrespectful things I've said about W is that he's too old for her and this sort of relationship is very toxic.

After ignoring my comments, she continued to show me more texts and full on IMAGES they've shown to each other. She proudly put those into my head and talked about how sweet and hot he was. I don't mean to sound creepy, but I noticed most of her images she wore tails and ears. That's when I realized that A is not an actual kemonomimi, and actually participates in pet-play. Something else to note is that (if you can't already tell) A is super mentally ill and has already been to a mental hospital. She often, very quickly, switches up on me and goes from happy to uncomfortable. I've often tried getting her to explain anything I've done, but she refuses to talk to me when theres anybody else around. I think she participates in pet-play due to things that happened during her childhood, which I am NOT sharing.

Should I get rid of my friend, even if this sort of thing is therapeutic to her in any way? Also, can I file a sexual harassment report to my school about this? I don't want to tell on her dating A or W, even if their relationship scares me. But what A showed and did to me made me very uncomfortable and idk what to do anymore.