If she changed the Apple ID, no one is going to be able to help you with that. I would say part of your issue is with Boost mobile if they allowed her to make all these changes to your account. I'd be rather upset with them right now. However at this point, there is no one that can assist you to get her Apple ID, even Apple would not be able to help.
I have Boost mobile. I bought 2 iPhone 6plus devices in September of 2016. I allowed my niece to use one phone but only if she was living with me and going to school. She ran away, stole both phones. Changed all account info at the Boost mobile store and they can't assist me because I don't know pin even though phones are in my name. I was able to get my phone back, went to Apple to replace screen. They said Apple care wasn't in my name. I went to a phone repair shop, got screen repaired. Now I'm trying to locate other phone but don't know her Apple id
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Stolen phone with Boost mobile on family plan. Apple ID changed account info changed