We've just "converted" my 82 year old mother who has dementia. She sounds like your relative, i.e. constantly going to the building society to withdraw cash and ask for cheques, she'd withdraw a shed load of cash then wonder round town with it, go to the post office to pay some bills, and post cheques for others.
We had to "convert" her because she was losing too much cash. Her passbook would show, say, £500 withdrawn, but a couple of days later, she'd have nothing in her purse. Heaven knows where it went. Probably in the bin as she had a paranoia about people coming into her house stealing things (they didn't) and would hide things in weird places that she'd forget - it wouldn't surprise us if she hid cash in a corn flakes box and then threw it out, or it disintegrated if she hid it in clothes which then went into the washing machine, etc!
We closed her b/s accounts and opened a simple cash card account with the Halifax, got all her pensions paid into it and set up direct debits to pay all her bills. It was hard work, but she "got it" in the end, as we took her shopping and kept forcing her to use the debit card to pay, and eventually, she got the message, managed to remember the Pin number, etc. Now she literally doesn't remember cash at all, she just gets her card out automatically! And with it being set up for nearly everything to happen automatically, she never looks at the account itself (we check the bank statements to check nothing untowards is happening - she's incapable of reading a bank statement).
Major success and we've managed to stop the "leakage" of her cash.
Most of the oldest people have someone (friends, relatives, etc) who helps them with things, and dealing with money is just another thing they need help with. It takes a bit of time and perseverence but is well worth it.