Having been stalked - by the legal definition - document everything that has already occurred and take it to the police. Continue to document everything. Your son, as the person being stalked, should notify the girl in writing, by registered mail, that neither he nor anyone else in your family wish any further contact with this girl or any member of her family. These are the steps the police gave me - in the end, I had to file for a restraining order to get this guy to leave me alone. In retrospect, I really wish I had done it sooner. I was urged to go sooner, and didn't, because involving the police seemed so intensive. This was after our school resource officer (a police officer who is stationed in our school) had fielded a phone call and several attempts by this guy to contact me at school - and even then, it took him showing up at my house again before I finally filed - and that caused both the responding officer at 911 and the jduge to ask what took me so long to take action.
It's a hard thing to do, bringing legal action against anyone, much less someone with a psychiatric disorder (the guy stalking me was diagnosed with obsessive/compulsive disorder, adult ADD, and bipolar disorder) - but you need to protect yourselves. No matter what this girl thinks about your son being kept from her, no matter who she says told her what or what she has told her family, it is her illness talking. If the previous actions - quitting his job, dropping Facebook, changing his phone number, etc. have stopped her, then nothing other than a sharp, clean, break - as denoted by legal action, if necessary - is going to stop her. If she truly has a psychiatric diagnosis, and violates the restraining order, chances are good she will be mandated into therapy; if she is not formally diagnosed, then any lawyer worth even a few cents will argue based on her need for psychiatric assessment, and she will likely be mandated into therapy, either in or out of the jail system. But however hard it may be to take such action, you need to protect your family.
Good luck to you all.