All he needs to do is set up an NFS export from his ubuntu PC, and then mount that on his mac (I don't have any mac stuff, so can't advise in detail, wait! Here
The fact that 'buntu is using ext4 doesn't matter, as 'buntu will read the filesystem, then send the file over the network. As such, the file is just "network traffic".
Edit: Ahhh! You mean the 'buntu that is on his mac's HDD?
Not possible until mac develops support for ext4. Maybe you could do something with a "virtual machine"?