Buck up and tell her to back off. Some situations call for a little meanness. People will walk all over you otherwise.
Offering to help someone is one thing, but hovering and gushing information (especially to relative strangers) is past her boundaries, and you shouldn't have to put up with it.
Horses aren't stupid. They know the difference between a "good job" pat and a "back off" smack. Your horse will survive being patted. If that's the worst thing that ever happens to him, he's got a darn good life.