Money is money, this client didn’t take their money away, right? That’s the only thing that matters. De-escalate the situation and have a nanny comfort the idiot parting with his money - play it smarter, silo off communication from the the rest of the team and have it go through the nanny (and pay the nanny a bonus later for managing the situation). If the current point of contact is not working, have them become the bad cop, and throw in a good cop temporarily. Have bad cop always deliver the obvious requirements, have good cop deliver their special requests. In fact, reset the relationship with the good cop and just have him/her straight up appeal to the client ‘we believe this project can be handled better’.
But don’t send away the money if it’s not walking away on it’s own.