If you live anywhere an MS Therapy Centre, they will almost certainly have tailored courses - if your mum has real mobility problems, check out Feldenkrais.
I do chair based Pilates - very good for balance and has the added benefit of not being too energetic, so you don’t get too hot. Mine isn’t an MS-specific class, it’s just over 50s, so we’re all in varying degrees of mobility.
Yoga is another recommended regime if your mum doesn’t cope well with heat.
If you have a reasonably active local MS Society, get involved - ours have organised a specifically designed exercise courses - whatever you’re interested in - ours do walking football, amongst other things, for example. And they are quite happy to have a plus one along, so your Mum can take you along too if that would help.
Your mum’s MS nurse will probably be able to get you on the list for neuro physio at the very least, and will probably know of more.
Exercise generally - just don’t overdo it, and your mum will find it beneficial - apart from anything else, it releases endorphins, which are the body’s natural happy juice - that’s why some people get addicted to exercise - all of which will make her feel better.
You don’t need to do a prescribed exercise regime, just do whatever suits your mum - it’s all good. But if she enjoys her exercise, she’s more likely to do it, do bear that in mind.