It is quite common for [puppy mill rescues to be used to "going" in their cage / crate. And it is common for them to have few or even no teeth, as the mill owner does not bother with vetting the parents.

Puppy mill rescues CAN become the wonderful dogs they were born to be. Your mom should go into this realizing that it will take a bit of time, but lots of patience, love and a steady routien will bring the dog around.

Puppy mill rescues have never felt grass; have never learned what stairs are; and seldom had a kindly human touch. The whole world is new to them. That can be scary, so it takes some time. Often what helps them is having a socialized dog living in the home already, so they can see behaviour that is acceptable and which does not result in a reprimand. Then they start to copy that behaviour; it looks FUN! They also always need a fenced in yard as they are not keen on sticking around to see what the human will come up with next.

Belly bands are quite common for the male puppy mill rescues; it saves the floor, carpet, furniture and sanity! (The females start out wearing bikinis for the same purpose.)

As for this shelter: do they let the other dogs out? Is it because this one is a puppy mill rescue and they don't have the time/ staffing to work with the dog? Or do all their dogs remain confined?

Will the shelter permit your mom to have the dog checked out by her vet before she actually commits to adopting him? That may be a good first step in this situation. And maybe talk to the vet about the shelter as well, see if there is any "word in the industry" about the place.

Let us know how this works out!