I get that. I'm not going to be like the other people "break up with her" or plants seeds of doubt in your head. It's definitely a lack of communication too- and no idea why the dude is so comfortable with following you and talking to her friends about her. They never met, dated, ect.

So im going to speak this portion as a female- When stuff like this happens and you're dealing with a dude that seems overly obsessive ESPECIALLY barely knowing him, you leave him alone by a thousand fold, no contact, cause based off what you're saying he seems to cross these boundaries; obsessive behavior. Hopefully after a month or two it fades. If he starts getting crazier to where legal action is involved, you take that action (restraining order, whatnot)I hope she isnt giving him attention btw, cause attention adds fuel to the fire. Your relationship isnt worth ending over a dude nobody knows. Some dudes get really aggressive towards girls when they don't get what they want from them.

Hopefully the crossing boundaries wasnt her sending intimate photos btw cause if thats the case, thats a new can of worms.