I have arthritis in my hip too. Mine wasn't diagnosed until it was severe and now I'm on the list for a hip replacement. I'll tell you what my experience is...
Diet: nothing seems to make my arthritis better or worse, except alcohol. I've cut that right down to almost none now, and that does seem to help.
Hydration: drinking lots of water seems to help. Def over 2L per day, prob closer to 3. I realised that I was restricting my fluids because it hurts me to get to the loo, so now I make myself drink. It has to be water, nothing else.
Supplements: I'm not very good at taking them, so I'm not the best person to advise... But one of the reasons I'm not good is that none of them seem to make any difference. The arthritis nurse practitioner said there's no clinical evidence that and food or supplement helps or worsens arthritis.
Heat: definitely helps. Being warm makes it better, being cold makes it worse. I wrap up, wear socks in bed sometimes, use wheat bags and I've joined my local pool/spa to spend as much time as poss in the sauna and jacuzzi. These help more than anything else, and I can't recommend them strongly enough!
Exercise and keeping mobile: I can't really walk at the mo, but I can swim. So I am, and it helps. Fidgeting helps when I'm sitting still, or I seize up!
Weight control: Easier said than done. I'm overweight. My weight has increased by 20kg since my arthritis got bad. And without a doubt, the heavier I am, the more my hip and knees hurt.
Painkillers: ibuprofen doesn't agree with me. Naproxen works better. I've heard that TENS machines can help too. :)
Good luck to your mum. :)