Your girlfriend needs to have a custody order worked out for her son, defining when and where ex husband can see his son. Until then he is going to use the boy as a pawn to punish your GF and feed his own ego, to the detriment of the boy. The kid has years of therapy ahead of him if your GF does not get a legal custody plan in place and some stability in the kid's life.

As for the ex-husband, ignore him, but don't block him on social media! Anything he says can be used against him in the custody hearing. Once the custody plan is in place there will be set times and dates for the boy to visit. If her ex violates the order get a restraining order against him (so he doesn't show up unannounced at sports practice for example), if he violates that call the cops. Some people are just narcissists just realize that and move on. Don't let this guy live rent-free in your head, that's what he wants.

But before anything there needs to be a judge ordered custody plan for the kid. Reach out to the lawyer she used in the divorce, or call your local bar association for a referral.