Hi Siara,
It's difficult to say about market size. I doubt if he'd make much money. But if your friend has the philosophy of wanting to help modellers then it is definitely worth doing. It does look a good product, and superdetailers would really appreciate the product.
By the way, the insulators would be great for the 72nd Type II kit. Modellers would, I think, want to get them for their Type IIs as well as Type VIIs.
Given the small size he'd cram a large number of insulators onto one brass sheet. So production costs per unit would be low.
I'll post a poll on the forum to ask others about this.
Cheers,
Dougie