The guy is saying "if you can't finish the job then let me know and I will hire someone else" . I'd sit and have a good stern talking to him. When they get this attitude they can have you dancing around like a puppet on a string.
If it is one thing that infuriates me it is people watching your jobs and rating them while you are still doing them.
When my painters get a customer like this I tell the customer to back off and let them finish. Then if there is a problem we will talk about it.
If they keep going I make sure I stay ahead of them financially.