It's certaily possible for the coatings to become damaged or to delaminate. If the outer protective layer(s) are damaged (by physical impact like an object hitting the front element or a scratch caused by larger particulates on the lens when it's rubbed with a cleaming cloth), moisture can penetrate the layers of coatings, and some of the antireflective layers are quite susceptible to moisture damage (not even water on the lens - humidity is enough). Use of inappropraite cleaning solution can damage coatings, even Scotch tape can cause microscopic lesions of the surface.
Possible explanation, but it was quite widespread over the front element (sorry, I didn't think to take a photo at the time). At first, I thought it was just moisture, but it wouldn't rub off; it also didn't look like your usual scratches: they weren't thin lines, but fat and speckled. It's strange because it's his newest lens (less than a year old) and none of the others are in this sort of state, despite similar usage.
UPDATE: I've just had a 'phone call from him (whilst typing this); Canon are blaming this on "excessive cleaning". The lens is just over 12 months old and apparently been cleaned with only a microfibre lens cloth... Time to buy UV filters for my lenses? Oh rats, the good ones are multi-coated too!!!