Robert, An interesting project and I wish you well with your endeavours, however please keep in mind that whilst it is not illegal to load ammunition for any type of firearm — provided you have the necessary UK certificate — and its for your own personal use.
It is however an offence to to reload ammunition for another party, even for free, unless you are CIP Proof house approved as an ammunition manufacturer where your ammunition is tested and approved by a CIP proof house, in the UK Birmingham or London but I believe the testing is largely done at Birmingham. All ammunition sold in the UK is marked as approved on the box.
Manufacturing the components i.e. cartridge cases may be a grey area, I have no idea but would suggest you contact Birmingham proof house, you will find them very helpful I'm sure.
I would also suggest your friend ensures that his gun is still "in proof" under he rules it was tested for – many old guns are way out of proof and thus unsafe for service unless re-proved at the proof house.
John