I think I might send a polite note to the new g/f and tell her that nothing would please you more than for your son to have a happy, healthy relationship with his father, but that the family court spelled out conditions for that to happen, and they require his father to do certain things. So while you appreciate her intentions, it's his father that needs to do some things to make that happen, not his son. You could say that until Dad does those things to make it safe for them to have that relationship, you would appreciate her respecting the court's decision.
I think that might accomplish a few things: You're a reasonable person who respects her good intentions but also suggesting she doesn't have the full story. It's neutral enough in content that you don't come off like some wicked witch standing between a child and his father.
Whether it gets her to back off or not, it's hard to say, but it doesn't seem like it could hurt.