2006 Chevrolet Tacuma engine light went on, engine still running but all power died - towed

I have never had any problems with this car and sold it last week at 38K miles for next to nothing (£1.3K) basically because it is as uneconomical as they come. Having driven it until the hour before the buyer came (eBay) I can honestly say that this regularly serviced car has never broken down or had anything go wrong. He has contacted eBay to say the item was Significantly Not As Described and opened a case against me. He says he has driven less than 80 miles and he was driving back from the supermarket when the red engine light came on and all power died (engine still running). The guy who towed it said maybe head gasket but it is going in for a proper diagnostic tomorrow (currently "undriveable") and he clearly thinks I am accountable. I am gutted. Does any have any bright ideas assuming this story is true.