Will they be, if he was using POP, then the emails will have been downloaded from the server to his computer. I'm not so sure they will be on the server. If the service he uses allows for web access that's one way to see of they're there.yes, the mails are probably still on
If you simply disable the account - (Mail > Preferences > Accounts: Enable Account) you should be OK - it'll take the account offline but the mail archive remains. The danger is if you delete the actual mailbox(es) or remove the account from the account list.