Downhill walking issue - lower back?

Hi, I just wanted to query some difficulties my mother is having when she does a mountain walk. I’ve read about knee problems being common, but what has happened is her lower back starts aching and her legs start to wobble causing her to keep losing her balance and leaning backwards. We walked Pen Y Fan a couple of months ago and half way down hill I was having to hold onto her to stop her from falling backwards, as if she couldn’t stay on her feet. No problems whatsoever uphill but with a forthcoming mountain climb booked late September I wanted to look into this to try and avoid it happening but also to gauge whether it is advisable for her to continue. She is happy to book in with a physio for advice, I’m just not certain whether it his her legs or back that is the issue. Many thanks for any advice given, Amanda