I think you should draw his attention to the part in the owners manual where it states that the owner of the car should operate the A/C for around 30 minutes once a month to keep the system lubricated and operational - seeing as he hasn't used the A/C at all for 4 months, against Fords advice.
I wonder how happy he'll be when he finds out he has to sign a disclaimer at KwikFit/National etc to say that although they are re-gassing the A/C, they can't guarantee how long it'll last for!