I don't know from personal experience, but that caa grip does look good for the op's arthritis criteria. It's like a pistol grip version of a condom with interchangeable ribs (for her pleasure as well).
Seriously, it does look like it would help mitigate some of the hammering like force from recoil. Yeah, an ar15 don't recoil that bad, but where arthritis is the case it don't take much depending on the severity of the shooters arthritis. With all the interchangeable front and rear straps, you should be able to find a combo that works for you.
Also they used to make gloves that helped in reducing felt recoil for high powered pistol shooters back in the day. I'd try hogue first for these (iirc). I'd take a look at hogue' s grips as well for that matter.
Like I said, I'm just going off looks, not from personal experience. Just have thought along a similar train of thought before.