Question about signatures whilst someone is deemed to not have capacity - England

Hi all, I did a sperate much longer post about the main question I have, but as an aside I need to ask about something specific

My Father was deemed to lack capacity to make financial decisions as a result of a progressive condition (specifically dementia). As a result my Step-Mother and I were co-attorney's for health and finance.

During this period my Father's friend who was also acting as an editor on his book and self appointed agent signed a contract on his behalf. He admitted he did this in an email conversation with me.

We were neither aware at the time nor gave permission

My Father was incapable of giving permission for him to do so at the time

Is this legal? Is there likely any potential consequences attached? The main outcome was the payment my Father was entitled to was paid to the editor to be paid out to my Father. He's now holding this hostage demanding to keep a portion of it for various services he felt he rendered. Before we come back at him with a second and final demand I just want to understand where he would likely lie with this.