What an awful thing to have happened. Does your mum belong to any sort of professional body where she can go for advice? Is she insured? I'm guessing the dog was a shih tzu. It may be worth checking with the shih tzu breed society in your area to see if there could have been an ongoing condition that causes that to happen.
As I thought, the shih tzu is prone to certain eye conditions, some hereditary and some just because of the dog's shape. There is a particular condition called proptosis where the eye pops out and the eyelid closes behind it. Treatment must be started within 20 minutes or blindness is inevitable.
This site tells you more about it:
Eye Proptosis in Dogs
It sounds like the dog needs to experience some sort of trauma for it to occur - perhaps he hit his head as he ran out? It can also be caused by incorrect handling. Whatever, it doesn't seem to be something that will happen spontaneously. I think your mum needs to contact her insurance as it sounds like the sort of thing that could easily happen in a grooming environment and as such she should be insured against it.