Check the Outlook configuration on the laptop. Specifically where mail is delivered. If he’s set up mail delivery to a .PST file on the laptop it will take the mail from the Exchange server to the .PST every send/receive It doesn’t matter if the connection to the Exchange server is POP or MAPI (Exchange).
If he wants to be able to access his mail when not connected to the Exchange server set the Outlook (2003 or later) to use “cached mode” which will create a copy of his Exchange mailbox in a .OST file. Then import the .PST back into the mailbox.