Hiya, my Bridge Baby Max suddenly went blind as well, he had kidney and thyroid problems.
I'm not sure if it was complete sudden blindness, or just an onset over a few days, but his eyes were huge and black, and we were told his retinas had detached. He was prescribed 1/8 tablet of Istin every day to bring the blood pressure back down again.
Max was always and indoor cat using a litter tray, and he amazed me how well he adapted. The vet things he may have actually regained a bit of his sight as he did seem to be able to follow things, which not have been just through sound or senses alone. He learned to jump onto things still, and did pretty much everything normally, just a much slower and more cautious version of normal.
Sadly my boy passed away last month due to his kidneys, so I didn't get to see how much more sight might have come back
Best of luck with your Mum's cat, and hopefully he can learn to adapt to his new environment xxxx