I had a 'break' of nearly 25 years - I dived professionally on a search and rescue team, starting around 1979/80 for a couple of years, but then promotions meant I couldn't dive any more. That was all fairly extreme, cold water low/no visibility, hazardous entry and exits, overheads and plenty of health and safety risks.
I came back to diving six years ago and really did not feel confident to just jump in and go after just a scuba review, and that was diving in warm water, wetsuit/rash vest and 100' visibility
.
I made a conscious decision to retrain completely from scratch, rather than try to work out what had changed or I had forgotten, so I started out with CMAS doing their open water equivalent. Although I picked most of it up intuitively straight away, and the theory really was so straight forward almost just a refresher, what it did do was give me time in a controlled environment to break myself back in, learn to be comfortable and neutrally buoyant again, and gave me the reassurance that I did actually know everything I should know in order to be safe.
Personally I didn't stop there
because I wanted to push myself fully and prove I could do it in my 'later years'.
If I were in your mums shoes I would do an open water from scratch, but be honest and tell the instructor my background, treat those parts that come back easily as a refresher, and concentrate on things that either I don't know or remember or 'age' had made difficult or requiring of a different technique. At OW level it really doesn't matter too much which agency, PADI is fine, those basic level skills are pretty much the same across all agencies.
Provided everything goes well, and there is no reason why it would not, then you both at least know that your mum has refreshed or relearnt everything she needs in order to be safe.
I hope you have many happy years of diving together - P