As the title states and I have little knowledge about this camera I'm turning to the Canon users here on the forum to perhaps maybe help me in finding a solution or cause as
to why a 5DmkII seems to lose focus on and off or wont at all.

This is for a friend of mind so I've already asked some of the basic stuff like 'Check connectors on lens and cam, AF points, not using manual AF, single AF ect ect..

So apart from the obvious settings we tend to check, what else could it be?

Cheers for any enlightenment