It's not a "fake", it's absolutely, 100% genuine Mdina. Christine's right about colour variation, it depends on how deep the colour of glass melted in *that* pot was, the amount of silver chloride in it, and how that reacts with both the blue and the clear glass. Every piece of Mdina is different, and the colours are often very different too.
The silver chloride is horribly expensive nowadays, I know of no fake bits coming in from anywhere - I don't think it's raising good enough prices for anybody to be going to the expense of faking it.
This is a Michael Harris design, a highly desirable one, and it continued to be made for a while after he left. Your buyer got it quite cheaply - not long ago, these were regularly raising ~£75.
There are a lot of fake signatures now, some better than others. Many, you can tell just by the incorrect spelling of "Michael". There is an image of this style of "Cut-ice lollipop" on p.51 of Mark Hill's book; "Michael Harris: Mdina Glass & Isle of Wight Studio Glass". Interestingly, the image in the book has been taken under the same lighting conditions as this piece, so they do look very similar indeed. It will look quite different under different lighting conditions.
I can't advise you what to do about your buyer though. Send them here?
Would you need the word of Mark Hill or Ron Wheeler to give the attribution complete authority?
Your item is not misdescribed, it is absolutely correct.