If he wants to use the standard footrests and controls etc then it's usually possible to do something with a plate mounted on the original holes in the frame and the footrest hangers mounted lower on the same plate. But if he wants something a bit more special, then I would think someone like Harris or Spondon would produce something based on their standard stuff to the exact spec' needed.
Bradders has some riser plates that I'm sure could be swapped over to act as lowering ones. Or used as a reference for machining some. It can't be too hard with the right kit.
They did cause a problem with pushing the pegs out away from the brake lever though.
The ones Bradders has are Valtermoto ones that he had off me, I have another new set going for £30, I also have some other unknown Titanium adapters that are intended to raise the footrests without moving them backwards. I would have thought either of these sets or ones similar would allow the pegs to me moved downwards.
This is just another reason to get one of those new fangled bikes, as they have several adjustments available
Getting back to the original question though I would imagine it wouldn't be hard to knock something out yourself, or the likes of Harris etc probably do a kit anyways, Skidlids sounds like the best option for £30 though