Is the disk a DVD-RW? if so a DVD-rom drive cannot read DV-RW disks.
Is the DVD mounting at all? Are you sure He has a DVD Rom drive?
If it does mount, have you tried to drag the .dmg over the 'Disk Utility' app manually. Sometimes doubleclicking a .dmg causes stuffit expander to want to be loaded. and some version of stuffit can't open .dmg's
Stupid questions I know but you'd be suprised.