The TRX Suspension trainer is my recommendation for a total body workout. Seniors with arthritis just happens to be one population that benefits greatly from the unique qualities of the TRX.
TRX is an effective exercise tool for most body parts. It is safe and effective, even for people with multiple conditions, dysfunctions, and limitations. There are quite a few exercises that those with limited range of motion and functional deficits due to weakness can perform.
I assume you have already pointed out the possibility of a ball, Bosu, dumbbells and elastic resistance instead of a ‘piece of equipment’.
All equipment is designed to accommodate many fitness levels, so no matter what you recommend for her, there will always be ranges that she may never use.
I personally own both a Bowflex and a Total Gym (I have the Power Tower now and had the manual model before) and use the Power Tower a lot more for all clients. It starts at a very low resistance level. The only adjustments are the angle of the board and possibly some attachments. The Bowflex has also very low resistance but adjustments can be challenging for some people. My clients often comment how they wonder that I can remember it all (it’s really not much but that is their perspective).
The Total Gym is not difficult to use but people still have to be instructed in proper mechanics. That also applies to the adjustment of the Total Gym itself because the manual version has to be lifted from level to level.
I wish you good luck with your client.