He's running an Active Directory server on his home network, so it would be using his slow home internet connection to upload data.

Oh, right, I didn't consider that AD would only upload stuff when he logs in.

I don't quite get why doesn't he use something that syncs before he reaches the office, though. Setting up the machine to power-on automatically a couple hours earlier and syncing using Dropbox, Unison, etc is not that hard.