NTA

One of the most shocking things about this is that he's taking his BM's side!?

His BM is a classic raging Malignant Narcissist and she's used a textbook ploy of turning him against you, and sending him out to attack you on her behalf, in which he's now acting as her flying monkey.

Something tells me you and your BF are really (REALLY) are going to need all the help you can get to stop her for her attempt to destroy your relationship and cause you psychological pain and stress.

You should really start with this subreddit:

https://www.reddit.com/r/raisedbynarcissists/

Also check out Dr. Ramani's videos about people like your boyfriend's BM and how they manipulate, operate, and destroy other people's relationships:

https://www.youtube.com/c/DoctorRamani/videos

Your BF is going to be the one who really needs to educate himself the most on how he's being manipulated, and has already allowed the manipulation to turn him against you.

Hopefully his love for is stronger than that going forward.