How - Granting account priviledges

Hi Forum!
With the plan upgrade, I can now host 2 domains beside my own, so I’m thinking about hosting a domain for a friend. I’d like to give him complete control (admin, ftp, mail, etc.) over HIS domain but not over mine.
– What’s a good way to achieve that?

I imagine he should have a separate WebID for the Panel. I can then grant him access priviledges to my account - but I want to make sure I only grant him access to his domain in my account, so he can’t mess with my own domain. Any thoughts and comments will be most welcome!