So my friend just purchased a Macbook Air from Best Buy and the salesman claimed he works for apple and any type of hard case will make the laptop overheat and void the warranty, has anyone ever heard of this?

I’ve had over 7 Macbooks. Standard, pro, air, etc. All of them have used hard plastic upper and lower snap-on cases (With vents in the bottom) and not a single one has had a single issue overheating.