Breast buds or fatty tissue And puberty

Hello! My boyfriend’s daughter just turned 8 and is overweight by about 15 lbs. I’m not sure if that puts her as overweight or obese. I’ve read a few posts on here about girls starting puberty early especially when you rate overweight and I’ve done some research on it. My first question is how do you know if it’s breast buds or just fatty tissue? I’ve been telling him that we need to have a conversation with her about growing up and periods just in case something were to happen so she won’t freak out. He’s a bit uncomfortable about it and her mom won’t even take her to the doctor for checkups and still talks to her in that high pitched baby voice so I don’t think we can really depend on her explaining things well or wanting to and I want her to be prepared for when it does happen, whether it’s sooner or later. Any advice is appreciated.