[20F] looking for advice about Boyfriend [22M] girl best friend.

Dating for 3 years. My boyfriend [22M] has a girl best friend since high school. I am very uncomfortable with them being friends anymore because of some recent events.
Before me and my boyfriend [22M] started dating, he told me someone had shown a love letter he had written years to the girl best friend, showed it to girl best friends boyfriend and the girl best friend.
After that happened the girl best friend suddenly cut him off (I assume her bf wasn’t comfortable) but she didn’t tell [22M] why.
For 2 years she didn’t contact him. But Suddenly a year ago she messaged him wanting to be best friends again, I didn’t mind at the time,(She still has same bf).

Fast forward to a few months ago she send me a instagram dm that completely changed the whole situation for me, before that I was completely comfortable with their friendship, very confused why I even received it.
This is the message:
‘I just wanted to reach out maybe for the last time, because Ive been getting the vibe that you might be a bit uncomfortable with me hanging out with [22M] and I just want to clear the air. I respect your relationship with [22M] and l'd never cross any boundaries like that, and tbh I don't want to be seen as a girl who would! It's not me. I also respect you as his girlfriend.’

Maybe I’m overreacting but it feels like this message was aimed to create division in my relationship and honestly act like there was something going on.
Is it appropriate for me to ask him to stop being friends with her?

TLDR: After a two year cutoff, my boyfriend’s [22M] high school best friend suddenly re entered his life. I was fine with it until she recently DM’ed me saying she felt I was uncomfortable with their friendship, completely changing how I feel about her boundaries.

I have read the sub rules, thank you for any advice!