Father has potentially been exploited over a book deal - England

Hi everyone, my Father unfortunately has dementia. However back when he first moved into supported living a colleague of his claimed that my Father had a pre-existing agreement to publish a book. The book utilised my Father's work and in his field he was highly respected and had been previously published. The colleague would come over weekly to arrange the book and we mostly let it progress as my Father seemed to enjoy it. Fast forward six months or so and the book finally gets published.

My step-mother with whom I share a financial LPoA is in discussions with this colleague, up to this point everything had been fine. Frequent contact, all very friendly etc. However suddenly he became standoffish and revealed that as a matter of fact that my Father would not receive any payment at all as the colleagues editorial work meant that all the advance went to the colleague. My Step-mother was like "okay well show me the invoice and we can go from there", he then started alluding to the fact that my Father actually owed HIM money for all the work he had done. My Step-Mother then sent him a message saying "look I understand maybe the payment was less than expected, that's fine but can you just explain exactly how it was dispersed for {My Father}'s records". Radio silence, my Step Mother continued trying to reach out to him over 2023 with no response whatsoever. She didn't reveal this to me, her reasoning was that I was dealing with a lot (I had some pretty major personal issues at the time) and didn't want to overload me.

Fast forward to this week, my Father's health has sadly deteriorated and we're heading to the end, out of nowhere the colleague texts me demanding to know about my Father's health. He finishes the message with "I have no contact with {step mother} and have no desire to". I share this with her and she then tells me the above tale and comes with receipts demonstrating that what she said was true (I do absolutely believe her it's just good this isn't a recollection issue). What bothers me is that it appears this man exploited my Father's poor state of mind to rob him of money's due. We're considering reaching out to the publisher essentially asking what was dispersed for whom and when was any contract signed? Because frankly after a certain point my Dad wasn't legally allowed to sign his name on a napkin let alone a contract and if this... gentleman... exploited this state I have a duty to ensure that whatever can be recovered should be. I'm angry but I'm mostly upset someone would do this to him, I just want to ensure that he has everything he is owed from the book and he was not exploited and would be grateful if anyone had any advice or ideas beyond the above. Thank you in advance