I imagine that your customer did not follow the "if" part of mixture's post ("if you follow basic maintenance steps and common sense when on the internet") and had only picked up on the "then" part (then you don't need AV).
(I do have AV on the laptop which sometimes connects to the internet without going through a stealth mode NAT router, or at least not one under my control. Using a stealth mode NAT router which I trust, with only trusted boxes on my side of it, counts as "common sense when on the internet" to me.)