He's correct that high metallic paints can sit differently on plastic, but blending out won't help that.
What blending will help is if he's not used the correct colour, which I'd say is what has happened. If he's bought the paint in as you say, he'll just have gone for the standard shade rather than one of the variants. As some have mentioned, a lot of us bodyshops now have doodads to scan the existing paint (photospectrometer), which, along with software can narrow down to a much closer colour match. It can then be further tweaked by eye if really necessary, but computer matches are generally more than acceptable to most eyes.
If he buys his paint in, it's unlikely he'll have his own paint scheme/software/scanner, so that option is unavailable to him. It sounds like he'd rather flick the paint he has left over into the adjacent panels to disguise that fact that it isn't quite right.
I can sympathise with him a little as our job is a nightmare with some of the fancy modern colours, where everything from air temperature, humidity and application technique etc etc can alter the shade a little, but it helps a bunch if you start out with the right colour!
As a possible solution, see if his paint supplier has the necessary equipment to scan your car for a better match. It still won't guarantee a 100% invisible job, but it ought to be much closer than that one, which simply isn't acceptable in my opinion.
