Your sister likely bought a used or refurbished iPhone 14 Pro Max. Unless she bought directly from Apple, it is a roll of the dice buying a used phone.
Unauthorized dealers will often "boost" the battery numbers to make the battery look better than it really is.
If there is an option to return the phone for a refund, that would be my recommendation. Otherwise, all that you can do is keep an eye on things and hope that the battery decline will slow down.....so you can avoid having to purchase a new battery in the next 6-8 months.
The document linked below makes a frequent appearance on this support site.
The All Too Common SAD Reality of Buying … - Apple Community