icloud help

Hello

i have abig problem on my phone

i bought an iphone 7 from a girl , she told me that she forgot the icloud password and told me to never format the phone

soo along time ago i had problems with my big brother he knew my phone's icloud soo to get revenge of me he format my phone and delete all the date u got what i mean

now the phone is like useless , i dont have contacte with that girl , the phone is not cheap and im too poor to buy a new one im using my little sister's phone

pls help me i realy like my iphone and thats why i wanna une it , pls help me i know her email and i have the phone's IMEI

Thank you

Omar.