You can't stop her unhinged behaviour on SM, but it's highly unlikely to do her any favours. The best thing you can do is not respond or react in any way, difficult though it is. They are both getting a kick out of hurting you. Take that power away from them, and your advice to what is a truly appropriate response from your lawyers only. Don't waste your time fighting such pettiness.
As for the rest of this sorry narrative, so he met her when she was just seventeen, and she's been waiting around on the periphery of his life for an entire decade whilst he plays happy families with his wife and son. I find this so unutterably pathetic that I'd even feel pity for her, were she not deliberately now rubbing your face in it by setting herself up as your son's 'mother' and claiming to be rescuing him. That is such disgusting, needlessly spiteful behaviour that it's difficult to imagine how much lower a person can sink. But looking beyond the spite, it's sad.
This silly woman has expended the best years of her youth on a married man and is now crowing because she's finally walked away with her 'prize'. Keep in mind, though, that he is the one who firstly took advantage of her youth and naivety, and secondly is egging her on in this childish behaviour.
You'll be free, able to process this and having moved on with your life by the time she discovers that this prince among men is hardly the prize she imagined. And it's well-evidenced that he really isn't. He's already treated both you and she with unimaginable callousness for a whole ten years. He's let her waste some of her best years whilst he kept his cake, ate it and made tiramisu out of it as well, until finally, he's taken the insulting timescale of a decade to make up his mind he wants her.
Her embarrassing odyssey is unlikely to bring her any ultimate happiness. Any woman with an ounce of pride would have been long gone. I hope all this is some comfort to you, OP, as you come to terms with what's been done to you. You really are better off rid.