AITA for telling my best friend’s gf she’s toxic?

I F (24) have been close friends with M (24) for 8 years, we even consider each other as “brother” and “sister”, but last Thursday I had enough and messaged his gf, telling her she needed to change her behaviour, be more mature and work on herself or end the relationship before things continue to get worse.

He has been dating F (20) for almost a year and throughout the months, he has expressed to me numerous times that he’s finding the relationship very difficult and hard work.

We’ve spoken a lot about how he shouldn’t accept her behaviour and that it is abusive. He always said he would speak to her about it, but never did and made excuses as to why she treated him that way (e.g she has undiagnosed BPD?).

From what he has told me she:

• Uses silent treatment, sometimes for days if he upsets her or they have a disagreement, then love bombs him once he begs her to speak to him again/apologises etc.

• Wants to know where he is and what he’s doing; gets upset/jealous if he doesn’t explain e.g he told me not to post any stories on Snapchat with him in it because he didn’t tell her he was hanging out with me and she would be annoyed

• Gaslights or manipulates his understanding of events due to her own personal interpretation e.g he complimented an actress’ performance in a movie, she said he just objectifies women and that she’s an accessory, when the compliment wasn’t about looks

• Makes herself the centre of attention/any argument and does not understand things cannot revolve around her e.g on holiday, friend broke his phone and could not message her at 12 for her bday. He used my phone instead but this was not acceptable and she tried to break up with him because of this.

This behaviour seems to be normalised or brushed over by them both, but I am pretty sure she knows exactly what she’s doing and he is too afraid to challenge her because he loves her and doesn’t want to upset her.

They both said they were disgusted with me and that I shouldn’t have said anything as it’s their relationship, not mine.

I’ve now lost my best friend as he said he doesn’t want to talk to me again. I do feel guilty about messaging her, however I genuinely was trying to stop things from getting worse between them.

I knew she would be angry but anything I said was honest and truthful: I simply called out her behaviour and asked her to work on things, hoping she would.

I couldn’t deal with it any longer on my behalf as I felt I was partly responsible for letting things spiral, when I could intervene, as nobody did the same for me when I was in an abusive relationship and my friend was always asking me for advice.