webmail to webmail.
In that case, all that I can come up with is forwarding, or as Tinjaw advised, set it up as IMAP at first. Then after all are on the server see if you can convert to POP. The folks at Fusemail may also be able to assist you with this. I use Fastmail and I can ask the dev's for this kind of help - sometimes thay can, sometimes not. I have heard that Fusemail has relatively "friendly" developers. Can't hurt to write them and see if they can help.
Does he have a lot of saved messages? Forwarding them as attachments would be the best way, but that could be done more easily if you can download them first, maybe into Outlook Express.