How to make my boyfriends daughter feel pretty

My boyfriend has a 13 year old daughter named Charlotte. I made a post before because his ex was messaging me on different accounts and stuff. His daughters face is burns on one side because his ex got drunk and mad at her and dumped hot oil on her face when she was younger. That's what lead to my boyfriend getting his one and only DV charge because he blacked out when he saw what happened and his ex was charged with child abuse. Charlotte gets made fun of at school and gets called Shireen from game of thrones. The girl with the stuff in her face. Her middle school is having homecoming week and some boy gave her a letter asking if he could just take half of her to the dance. My boyfriend wanted to speak to the school but she said not to. I spoke to her and she said she's used to it and she's accepted she's not pretty. I want to make her feel pretty because even with the burn she is so pretty. How can I build that self esteem?