I would advice her to go onto AKC's website and go in under hmmmm I think it says "dog owners to be" or something like that. From there she can choose breeder referal. The people who are working as referals for the different breeds are not supposed to be breeders themselves, so no lobbying for their own dogs.
But the list she gets from them should only be breeders who follow all the ethical and other rules the breed has. When it comes to Bullmastiffs, I would reaserch very carefully before I got one, don't get me wrong, I love them dearly own them myself, but I have also seen more than a few of them, that can be a bit too protective. If that's what she is looking for thats okay, but for a family dog, I would make sure I research the lines to make sure they are not too "sharp" or overly protective.
Marit