I used to be in HR. I had people ask me all the time to "help them" get a job, and interview, review their resume, give them contacts, etc.
I think at this point you should be honest with your friend. Tell her that YOU DID your end. You did forward his resume, and be honest about the feedback.
Tell her if you are willing to help him any more, that he would need to get in touch with you directly. And that you are happy to give him some professional advice, but that frankly, it's not professional for Mom to get involved any more than she already has.
Tell her that you understand she's just trying to help her son. But that at this point, he needs to do THIS for himself. Mom's pushing, prodding and asking friends for help is not the way to get her son off into the adult world.
If you don't want to be involved in helping him anymore, say that too. Tell her you did what you said you would, that you forwarded along his resume and it didn't turn into anything. Sorry, but that's the reality of the market these days. Then change the focus, "Where did your son apply? What interviews has he gotten?"...basically, what is HE doing to help HIMSELF? My guess is that will probably shut her up and get her off on some tangent about how frustrated SHE is that he doesn't have a job yet.