This guy was my best friend of 4-5 years. She was friends with him before we started dating. He introduced us and was the reason me and her are together.

They talk once every 2/3 weeks.

He recently acted very shadily as she told me some things that he said about me when he was angry. Also he has told her on multiple occasions that he thinks she should end it with me because he doesn't think she'll have time for a relationship b/c of school (Not something a bro would ever do, unless maybe if the guy cheated). He also would always undermine the relationship, prodding her why she thinks it'll work, questioning it's long-term potential, etc. Basically never having my back. On one another occasion she and I had a fight and she told him and she said she doesn't know if I truly love her. He implied that I probably don't love her and that people say 'ILY' all the time without meaning it. This is my best friend of 5 years, I've never wronged him in any way. This is not friend-like behavior.

All the backstabbing, relationship undermining is enough for me. My friends are all 'what the fuck are you still friends with him for.' I'm ending it.

I told the girl and she was with me 100%. She said she was surprised he threw me, his friend of 5 years, under the bus just like that, and that even she wonders if she can trust him sometimes. She said she can totally see why I was pissed, and she would be pissed if that happened to her.

So she asked "Do you want me to stop talking to him" and I said no because I don't want to be controlling. She said she would understand because it's him and what he did.

They talk once every 2 or 3 weeks, been friends for about a year. 0 romantic interest from her side, he's not the good looking type and she's the kind of girl to be nice to everyone/has one or two very ugly guy friends as well.

TL;DR: My best friend is mutual friend of LTR and I (they were friends before). He told her it's probably better to break up with me, implied I don't love her, etc. multiple times. GF supportive of my feelings, asked if I want her to stop talking to him. Said no at the time, but now I processed it and am about to give her the yes because I'm not letting my friend step all over me.

I'm going to tell her to stop talking to him, and block him on everything. What should I know? Any potential repercussions?

Anything I should be ready for? Just anything about RP theory that applies to this.

Thank you all very much.