I've got a hip replacement (that I just fractured about 6 weeks ago- another story-)-but this thread is something I follow. Mine is a large head design (56mm) so dislocation isn't an issue, but I've got other weakpoints in my hip design.
Acetabular cup alignment can have a big impact on stability. If there is impingement,
I hope the 2mm increase in size makes it work. There are all sorts of offset cup liner shapes to help with keeping the ball in the socket though.
Before the surgery, tell him to remember to get out his dremel tool and grind on the cup until things don't impinge. Then insert the liner. 
Asked in jest, it's serious question about alignment though. Glad as I am they found the problem, I would ask if going from 36 to 40 is going to address the impingement issue, or just mask it somewhat. Most replacements were only 28mm from 'back in the day'
iirc- 36mm was considered fairy large 30 years ago and was not routinely used due to liner wear. Highly crosslinked liners lowered wear to the point were the 36mm was considered usable from a wear standpoint and desirable to mitigate dislocation risk over the 28mm.
edit-
Glen you are supposed to keep the implant in at night. It's not something you put on your nightstand. 
Good luck.
Ron