But, a vent... I stopped paying attention to my social media last October. Just felt it was better to stay away.
It's usually a good idea to keep social media use at minimum. I deactivated my accounts for a year when I was suffering following my break up. Not that I was addicted to it, but thought maybe it's still a distractment from real life. During that period I got more social, made new friends, even a new best buddy and planned an awesome trip for myself.
And lo and behold in my "friend requests" is a friend of his girlfriend. I'd already deleted her before, so clearly she isn't giving up.
Maybe she gets a massive high on being righteous or playing a good friend or something. F her. Other people's relationships are other people's relationships. You seem to have this wisdom, but she doesn't. I think Once Removed's advice is good here, reject first and then block if it doesn't help.
Again, this is just a vent. I know there are many others who need help more than me here.
We all need help all the time.