Yeah, my Dad is hard of hearing. His hearing started declining in his late forties due to a rare disorder and it is basically gone now that he's nearly 70.
Firstly, please encourage your mum to do what she can, medically. Often deafness is either just incurable or functionally untreatable, but help her get the hearing aids money can buy because even the slightest improvement can make a big difference.
Secondly, understand that deafness can be super socially isolating. My father now discloses his deafness right away (because it's dumb not to), but for years people treated him like a total moron (notably, he's also visible minority) because he had trouble hearing them. I really wish we had spent more time socialising him during the early years, because he used to be an affable, popular guy, but literally not being able to have conversations with people have turned him into a social recluse.
Finally, be thoughtful and compassionate and patient. Your mum will probably need to pick up some level of lip reading, but that may take her years to learn. Make sure to face her when you're talking, and talk to her slowly and at a volume she can hear at (you may have to stop going to restaurants together, because you will often have to basically yell). Finish your thoughts as well - something my father really hates is when people give up halfway and just go "nevermind" when he's having trouble hearing them.
Anyway, I'm sorry your mother is growing through this. My father has had a really hard time adjusting to being hard of hearing, and the biggest thing I wish I had done differently is just be kinder to him, especially at the beginning. (I try harder now, but it's tough due to how closed-off he has become - a bit of a long story.) There's a lot of other detailed stuff you can pick up from a pamphlet on hearing disorders (and please do properly educate yourself), but yeah, above all else, treat your mum like the capable, intelligent person that she is and communicate with her lots so she doesn't lose practice.
Good luck, sincerely.