From many reports across the internet it appears to work fine provided you have the correct A20 BIOS, despite it not being on the official support list. Someone from Dell on the Dell forums even says it was validated to work.
That said if your friend already has a Q6600 he could do a BSEL padmod to 1333FSB and end up with exactly the same 3.0GHz as the Q9650, just with ~40w more power consumption and 4MB less cache.