If you have a bit of money to spend, I would go to a department store with a personal shopping service.
First, I would get a bra fitting done, and buy maybe 3. That will keep her going for a bit, and they are easy to hand wash. Also, some comfy knickers.
Then on to the personal shopper.
I would get nice fitting jeans, and the personal shopper should be able to advise on nice ones for her figure, and a couple of casual tops to go with. They can play around with tops that suit her shape, as well. I would also get her something a bit more formal, like a skirt or some trousers, and a blouse, or a dress suitable for job interviews, or other more formal settings. Maybe a blazer. But just the one outfit at this stage. But something she feels quite smart and confident in. Then a sweater, and a light coat, or rain coat.
For shoes, get a pair of funky trainers, and a pair of smarter shoes which work with both the more formal outfit and the jeans.
Once she has those clothes that look lovely on her, she can go out and get cheaper things from time to time, and experiment with her own style as others have suggested.
But some guidance to start with, so she gets clothes she looks and feels good in would be a really good start.
And
for your lovely mum, and for
for you for being such a loving daughter to her.