one thing iv'e learn't being self employed is some customer couldn't give a toss about how much they mess you about, and couldn't care less if it cost you time or money..
You want to be careful how much info you give back to the customer because I fear it will filter back down to the plumber and be used for his benefit, also sounds very much like the customer has been convinced/taken in by the 'charms of the plumberI've filled his slot now so happier days. I've told him its not just for water resisting, he needs the boards for the weight! Told him to watch out for dot n dabbing tiles. If anything I'm gonna make him graft for his rock bottom price !!!!
Or burn his house down!
You want to be careful how much info you give back to the customer because I fear it will filter back down to the plumber and be used for his benefit, also sounds very much like the customer has been convinced/taken in by the 'charms of the plumber