Cranberry juice has been proven to have no special properties

- it was a fad a few years ago

I would give Mum some wipes in a packet with the words "These are new on the Market, I find them really useful" ...... also, does your Mum use panty liners because these can cause irritation and infection if not changed often enough.
Like a cat which gets too old to lick itself behind the neck and shoulder blades, maybe your Mum is stiff in the shoulders or worried about leaning forwards to wipe properly. I know that many Indian ladies always have a special flannel for bottom use

which they use after each visit rather than loo paper. Another suggestion perhaps?