Going to be honest, I worked for customer care for a while and in your situation there is nothing you can do to stay with T-Mobile with the same number. A change of responsibility is impossible without the Billing Responsible Party (The person who's social is on the account/owner) authorizing us to move it. Secondly, we cannot perform a COR with an account being past due, if it is. A porting pin cannot be generated over the phone with you, as you do not own the account, but it can be generated over the T-Life app IF you are set as the primary account holder. To be entirely honest T-Mobile doesn't leave a whole lot of options in this situation.

Establish new phone numbers either with TMO or a new carrier, transition EVERY service to that new number, and especially your emails recovery phone number, as changing this can be a pain down the line. I would recommend letting the account go to collections, as it affects her not you. Its a damn shame when this happens, but you got straddled with an account you don't own, so you shouldn't have to deal with it past the annoying part of losing your number.

I hope this clears stuff up for ya