RE: My friend believes paying more guarantees more quality—a reverse of my lifestyle.

There is a saying here in Venezuela, I don't know if they use it in other countries, that says: "Music pays, it doesn't sound". It's like the case of your friend, he paid in advance for the painting job, and they did it reluctantly. You should never pay in full for a job like that, you always pay an advance, so that when you finish it, if it's not well done, you can complain so that they can do it better, otherwise, you don't pay the rest. Luckily your friend has someone to advise him, I hope you both do well.