My iPhone 15 Pro Max battery health is dropping fast

Yours should be 94%, so it’s very close. Your friend lucked out and got a device with a battery that exceeds specifications.

A battery is a chemical device, and chemistry is generally pretty variable and uncertain, as well as being analog, not digital. Apple specs the battery capacity to remain above 80% for 500 full charge cycles (1,000 for iPhone 15 and newer), but that is a minimum requirement; there is no published maximum expected capacity. So sometimes batteries will perform much better than that minimum specification, and sometimes the change in maximum capacity won't be linear. There is no way to predict in advance what the real-life performance of any specific battery will be.