Anyone with back problems needs something with a bit of support but also a softer comfort layer.
At the weight provided she would be matched to a Medium spring as this is solely determined by a persons body weight.
I would guide her to the Artisan Naturals or the Artisan Bespoke 004 mattress. The Naturals is the true medium feeling model in it’s comfort layers whereas the Bespoke 004 is still considered to be a medium feel but is softer than the Naturals.
We would need to know what she prefers the top layers to feel like and whether she is a side sleeper as that can make a difference.
I hope this has answered your questions fully but if you do have any further queries then please don’t hesitate to contact the office on 0161 945 3757.