Does she have the right to know that he has a son?

You're not focusing on the issue at hand. The issue isn't whether his current girlfriend should know. That has nothing to do with you, the mother of his child or his child. I would be more concerned about going to court and getting it ordered that he take a paternity test to establish him as the father. He needs to provide financially for the child at the very least. You can't force him to be apart of the child's life, although as a father, I don't see how anyone could not want to. Your friend needs to get a lawyer. Good luck.