Update from this post: https://www.reddit.com/r/asktrp/comments/87rj41/my_exltr_has_made_friends_with_my_current_openltr/

So my ex-ltr became friends with new-ltr (we're all 17/18), i believe in order to sabotage my new happier relationship, And has now invited the new girl over to her house. Who fucking knows what a crazy girl will do to the new girl that she's infinitely jealous of. I don't know how to bring this up to new ltr. She doesnt quite realise whats going on but I know its basically mate guarding if I tell her not to go, even if its for her benefit.

What the hell do i do in a situation like this?

(Also to clarify - new girl is my girlfriend, but it is an open relationship and im spinning other plates. I dont know if she is, and nor will i ask)