Pulling UNC Path from a mapped drive

Good morning all,

I have a need to pull the UNC path from a client's mapped drives, so
that my web server can go to the appropriate file server to retrieve
the file. Anyone know of a way to do this?? I know it can be done
server side, but I need the client's mapped drives.

Eg:

G:\file.txt

translated to:

\\server1\share\file.txt

Thanks in advance,

Matt Bailey