For the company you will purchase, you will point their MX record to your A record. It is more important to know if you will have people trying to send with the ‘old’ email address. The reverse DNS setting could be a problem if not associated with your A record.
Internally, you would want to add the ‘old email’ addrss to the appropriate individuals exchange email addresses. Make sure to set the appropriate address as Primary.
It sounds like he needs to use the ADMT tool to migrate the users from the new domain into his current domain. After that he can use the Exchange Migration Tool to move their mailboxes and their X500 records. Finally he needs to change the public MX record for the new domain so that emails sent to firstname.lastname@example.org will be delivered properly to the Exchange server.
Write again if you need more details on the process.