Doesn't matter where you physically file them. You will have to send then to the region that handles CA taxes if you file by mail, but you can file electronically just as easily from the UK as the US.
Most refunds go through electronically into a US bank account, so you would use whatever address you have for your US bank accounts. If you really want the check mailed to you, you can use whatever address you wish.
Don't forget because he has been a green card resident for > 6 years, he must continue to file US taxes now for the next 10 years. You have to continue for life (unless you renounce your US citizenship).