Hrm, I believe Tesla's warranty is "8 years or 120,000 miles, whichever comes first, with minimum 70% retention of Battery capacity over the warranty period.",

Hrm, I believe Tesla's warranty is "8 years or 120,000 miles, whichever comes first, with minimum 70% retention of Battery capacity over the warranty period.", so your friend should look into getting a new Tesla! Also, and I'd love if someone could dig up some data on this, the X and S used an older battery type that has since been replaced by a new chemistry and assembly which degrades quite a lot less than the older nickel based batteries. TLDR: I think battery degradation varies a lot by make/model/year.