Since their outside mail address is the same as their inside address...you'll have to be creative. They could setup a Yahoo (or other free pop3 email account) then in SBS you create a mail enabled contact for that person with the new mail account. Then in Exchange you can set the mailbox to forward all mail for that person to the mail enabled contact.
So even though you don't have a static IP, you could setup Dynamic DNS and have them get their mail via OWA