My friend was happy for me to manage the address on his behalf, but it's been so long now that I've forgotten the password. I could create a new sub-account for his (very) occasional use, perhaps sending a "dummy" message from time to time to keep it active, but it would be more convenient to re-activate the existing (suspended) one if that's possible.
May I suggest you create a Gmail address for your friend. That way it won't be tied to your account or BTnternet as your ISP if you change.
To keep the gmail (and btinternet) account active - just login to them at least every month. With something like Thunderbird, you can check all your accounts (including his) at the same time without remembering to login every so often.