Regarding metformin particularly - it's widely prescribed in Switzerland for diabetes and pretty cheap, so if your friend has Swiss insurance, it might not be worth the hassle to get it from the UK, or have it sent to Germany. Although there might be other uses, reasons to get it from the UK.

I take a prescription drug that I still get from the US, prescribed by a US doctor and sent to me from a US pharmacy, and paid for out of pocket. I have no trouble getting the medication from the US, it comes to me marked "for personal use." Of course, I have no idea how prescriptions, etc. work in the UK so I don't know if that's another option for your friend.