Your friend's Tesla did not drop in resale value because of the battery. If anything a new battery would make those who fear battery degradation more willing to buy this used car.

It dropped because Teslas in general have a high maintenance cost associated and a high insurance cost (because insurers also see it as costly to maintain). High maintenance cost tends to sink a car's resale value.

I know the car does not need oil changes or have the possibility of various failure modes an ICE car has, but when it does have an issue, like a dent or a scrape, etc. the cost to repair is much higher.