I wonder what kind of dog she has. Probably something small.
My family has always had dogs, usually larger breeds, and they've always been great guard dogs. None of them have ever been to a dog trainer. They just act instinctually to protect their family. We've had burglars scared away by them, one of which even broke a door to the point that he could get in, but never made it inside. I don't think actual guard dog training is necessarily needed, just a dog big enough to pose a threat, and have been treated well enough that he'll want to protect you.