OP, a thought just occurred to me. Assuming your son is on your cellphone plan, can’t you as the account-holder block numbers? I know I can on my children’s phones.
I would suggest to your son that he let you know once he has told the GF (and her parents, if he goes that route) that this will be his “final message” to her and that he will not be answering her texts and calls, and then you go in and block her number.
Agree that the break needs to be clean and final. Sorry the GF has emotional issues, but first of all, she is a minor, and this is not his responsibility to deal with.
Something similar did happen to one of mine. I was a thousand miles away and didn’t realize how bad it had gotten. Ds went to her parents, but they were in denial about her problems. Senior year he finally got a restraining order against her, which he felt was really important because they were relocating to the same city and he wanted a paper trail.
I wish I had realized how bad it was. He always was such a strong person that I had no idea he was in such an emotionally abusive relationship. She never threatened suicide, but she’d say things like, “I’m afraid to be alone,” implying that she might harm herself. If I had known then what I know now, I absolutely would have gotten involved, calling the parents myself. His last two years in college were a disaster, because of this on-again/off-again relationship. And, yes, it took some time for him to realize that once he set a boundary he had to stick to it. He was so afraid that if he didn’t unblock her number that she’d hurt herself, but, of course, that only taught her to amp up her episodes. In his defense, he thought by unblocking her it helped establish the electronic trail he needed to get the restraining order.
In good news, she never harmed herself. When they relocated, she contacted him a few times and showed up where he was a couple of times, but he hasn’t been bothered by her in almost a year.
Good luck. Ds hasn’t dated anyone since. It did a number on him, making him question his judgment.
Closing at OP’s request. She has received some great insight.