For some reason all the backrests now are pis* ant short.
Other than suggesting you get a Harley Electra Glide with armrests or a GoldWing, unless someone has an old bar that might work, you may have to fabricate something...
I hope she is seeing a doctor and taking appropriate medication for the osteo and arthritis.
I am also taking preventative stuff.
Good luck to both of you,
Mary