Are you sure that you want Remote Desktop ? For such distance helping you can use Remote Assistance.
Anyway, here's a page that sums up well both: Remote Desktop and Remote Assistance
An alternative for Remote Assistance is UltraVnc (or another VNC flavor).
Trying to unweave, unwind, unravel
And piece together the past and the future,
T. S. Eliot
I'm trying to access a remote computer to assist the user and although remote desktop is checked, I'm not allowed to assume control of the remote computer. Any suggestions are appreciated.