Who is he using for email now? His internet provider? Or are the emails associated with the domain names? If they are his internet provider he doesn't have to do anything. Those email addresses will remain and he will have his own domain email with his accounts.
If the accounts are associated with the domain he can talk to the provider and see if they can be redirected. Otherwise when he cancels the account there goes the email associated with it.
If he is really concerned I would get a gmail account and redirect any emails from where ever it is being hosted to the gmail account.