He does have a point to be fair. In the larger battery models, out of warranty battery replacement could be in the tens of thousands of pounds. While it isn't likely, it is a scary prospect.
Indeed, but the point is that it's no more scary than an out of warranty replacement for a knackered engine or gearbox; and given that an EV has neither of these, then on strength of probability, you're probably safer buying a used EV than you are a used conventional vehicle.