For any work I've had done at home, of this sort of value, I would always withhold a small percentage of payment to allow for "snagging", where things almost inevitably have to be put right. I have indeed withheld that percentage where things have been incorrect and the builder's not be able to remedy them. However I do make this clear when I sign the contract, that I will withhold the final 10% until I've inspected, and rectifications made if necessary.
Does your friend have all the requested amendments in writing, with appropriate sketches, and the customer's acceptance of them? Did he indeed get a signed contract for the original work? Does he have his terms and conditions for payment on the back of his contract documents?
As a customer I will never instruct work without documentation and I hope he has done the same and this is not all verbal instructions.
What does the customer want? - can the "problems" be rectified quickly without further cost, so your friend can get the balance of his money?