What you want to do is actually impossible. What will happen if you edit a part and write it to your disk and network drive while your friend is working on the same file? He may change it too and save it to network drive too. Your changes has gone.
I think PDM is your only chance.
But if you insist in your method there are a lot of software out there that can do this task for you.
http : // techsoftpl . com / backup / index . php (Take out the spaces)
This is one of those that I have used a while back.
Search for Realtime file-level mirroring from local NTFS to network drive in google. You will find a lot more.