You hit the nail in the head there and I wholeheartedly agree. Removing someone from your life who affects you mental well-being is setting healthy boundaries for yourself.
Making that decision is proactive because you’re making those recovery steps. That’s really important.
But that’s not what OP has done.
I quote
‘I messaged her saying that I don’t think I can be friends with her anymore if she is friends with them’
This is not moving away from a situation that is bad for your mental health as you explained. This is OP telling a person ‘I’m not doing x because you’re doing y’
Had OP removed this friend from her life to help her mental well being and set boundaries she would have done it without the threat. And that message is a threat.
‘She rang me crying and said she wouldn’t be friends with them anymore, and then that was that.’
By this point OPs action is now causing her best friend distress but OP is okay with that because she got her way.
‘Then the other day I saw that she was with that work group again. I messaged her again and said, I can’t be your friend, especially now that you lied to me as well.’
After this , if OP was really setting the healthy boundaries you talk of, she would have walked away and not made the comment above. Because if it’s about her she would save herself the aggro and run for the hills with her thought process to sort herself out. But now she’s pinpointing her BF for a second failure. 1. Being friends with another person 2. Lying.
Not sticking up for the BF but someone who is trying to remove a person from her life that is Upsetting her mental well-being sure is dragging it out. Because she’s not getting her way.
‘She just didn’t understand where I was coming from, and said it was wrong of me to make her choose between me and them.’
Friend reached out to explain how she feels but OP still doesn’t give a toss because it’s all about her and how she feels.
If she was trying to help her own mental state she really would have just walked away by now.
‘I can see why it looks like I’m making her choose, but at the same time it is very uncomfortable for me knowing that she is with them and friends with them. I wouldn’t do this to any of my friends and so I expect my friends not to do it to me. ‘
OP expects friend to behave in the way she would behave. This is controlling someone’s behaviour in a way OP would think is fit.
By all means walk away from something which is bad for your mental health but you don’t need a four part conversation to do it and threaten the person 2 times. You just walk away. This is OP trying to control Her friend and that is not okay to do to another person.
My Opinion.