Puppy mill dogs are not reared properly, so they do tend to have many problems. One of these is that they were raised living in their own excretions. This deprives them of the principle to NOT pee where they live. I think this is the case with your mom's dog.
Also, the dog may be displaying anger or dominance by doing this. When Cash was about 4 months old, he would *punish* me by peeing on my pillow and on my bed when I had left him alone too long. He was also from a bad breeder (I didn't know better then) and due to many behavioral issues, I enrolled him in a clicker training class. This miraculously took care of all the problems, including the peeing.
I would therefore suggest that your mom take her pup to training classes. The trainer will also be able to help with these particular issues if your mom asks him/her.
Your mom can also try using a diaper or panties to keep her from doing this, but I would go for the training first.
I hope this helps. I am afraid your mom is now paying dearly for her haste. Keep us updated. Good luck.
