Some of this is quite generational and I believe some of the people attacked (because it is an attack) are in the early stages of dementia. My mother fell prey to a "you have won" completion before we were aware there was anything wrong with her cognitive ability. Eventually the bank would not let her borrow any more - I think they did this at a much earlier stage than they had to.
A neighbour found her in tears and eventually I got to know. The neighbour phoned my brother (in America) who was sworn not to tell me; he phone his daughter (in the UK) who phoned my daughter and they both said they would not have any nonsense about not telling me and did so. Eventually we went to the bank with Mum and I believe they were very aware of what was going on - recognising the name of the company the cheques were made out to. Mum was very happy to let me try and sort it out after that but still wanted to reply to the "you have won" letters.
I wrote some legally sounding letters saying I would take them to court for preying on a vulnerable adult (mum never saw those) and the letters stopped for quite a while. I reckon she was hundreds if not thousands of pounds out of pocket. She was happy to give me and my daughter POA so I have been looking after her money ever since, paying all the bills etc., and sometime after that we got a diagnosis of dementia.
The letters did start up again and mum - no longer with access to a cheque book - was stuffing money in the envelopes and asking her mobile hairdresser and carers to post them. Luckily I go very frequently and they all felt able to let me know but, of course, they were in a very awkward position and it all took very careful handling. I had to make the decision to redirect mum's mail in the end, which s - probably one of the worst things I have had to do as she loved the post coming and the family sends her lots of postcards to keep in touch. The redirect letters from these companies have tailed off presumable because of a lack of response.
A couple of years ago when mum could reason more I asked her if she knew that x, y and z all thought these were scams. She said yes, she did. What is it that convinces you to reply was my next question. Because they are in black and white came the reply. Again, I believe, a generation acceptance that those in authority (printed letter) are right.
I feel so sad about the gentleman in the OP. The best hope is that the bank will intervene as I doubt he will be able not to believe it now he has bought into it.