Just a couple of years back, you would get replacable batteries, at least from “phone shops” with dedicated tools.
Designing a water tight enclosure which can be opened to replace a battery isn’t exactly rocket science. It doesn’t need to be as easy as a fairphone.
Sure some brands will do malicious conpliance and guess what, people will bux from other brands.
17 year old ? Still live with her parents ? It’s not a 2 persons relationship, but a 4 persons relationship, which make stuff way more complicated especially when it’s with parents and she isn’t independent yet.