Has fuck all to do with the suppliers mate they would have given them the same paint as the first time it was painted, you can get the odd problem but its normally to do with colour matching more than the products itself.

As said if the paint looked ok when it was put on the first time then when u saw it after it was likely to be solvent pop, which could actually be quite a few things tbh, putting on the coats too quickly after another, baking it at the wrong temp etc etc.
If they hadn't flatted out the popping and painted over it the second time it'll still come back as the paint would sink into the pinholes, as said sounds like the paint wasnt mixed properly the 2nd time either.

tbh 2k paint is fucking easy to use and im really surprised given the choice someone would prefer base and lacquer over it!! it takes twice as long as and by the time u buy the base then lac it costs more.

BUT if they want to do it let them mate and see what happens, but if poss make sure they strip all the paint thats had the problems off as it could still cause problems later on.

Althou they should know that so