yes.
If its his work BB, Laptop etc then bascially the mailbox lives on a server and is sync'd to all the devices. When you read the mail on a device you're really just looking at a copy of the mailbox, and then the status of what has/hasn't been read is also syncronised across all the devices.
For example, there is an Exchange server in the office, when the user receives an email a notification is sent out to all the devices which contains the header (To, From, Subject etc) and a small amount of the data, or maybe the whole thing.