R.M.
You just have to be straightforward. I just went through this same thing -- none of my friends went to my son's wedding, and when you think about it, that makes complete sense, because my friends are not his friends. A couple of my friends wanted to go, and I just had to tell them that it was not MY wedding and my son and his (now) wife got to choose their guests.
Just say "I'm sorry, only close friends and family of the bride and groom are invited, that's what the bride and groom have decided." And if she's going to get upset over that, then let her.
I seriously doubt her son is all that interested in going, anyway.