Extender problem

Posting on behalf of a friend whos is on holiday in Peru.

He has borrowed a Canon EF 1.4x II extender, and is trying to use it with a Canon 7D and 100-400 lens. If he mounts the lens on teh extender and then the extender on the camera, teh autofocus doesn't work any more. Both the mounts seem to click into place OK.

Should the extender work OK with theis camera and lens, and, if so, anyone have any idea what the problem is.

We'd be very grateful for any advice.