Cant you say yes if they tell you how theyre going to do it, and what risks there are and where your liability ends? I think you should keep engaged and fleshing out the terms and process. There may be a way to make everyone happy.
I agree. Maybe you could give them a scrap piece for them to try their hand at finishing. Make them finish it and bring it back for inspection before you agree to letting them finish the entire job.
It does not matter to me if the customer wants to finish it or not. Once they sign off and payment clears they can run over it with the truck if they want. That's their business...