Host for a customer who has his domain registered at DH


My friend has a domain registered with DH. I used to host his site on another host (and set the DNS at DH to point to that host), but now I would like to host it on my DH VPS. However, when I try to add his domain, I get the message “You can’t add that domain: you don’t have permission to add”.

Is it possible for me to do this? And if so, what do I need to do on my end, and what does he need to do on his end?