Alright. Problem isn't solved yet but I think their tech support actually figured out what the problem is this time but they aren't empowered to be able to fix it.
So apparently the fix for this problem is to make another account, add the original account to your xbox family thing with the new account as an organizer, and then give it time.
She's on the "Give it time" phase. If it doesn't work out she might have to play on a new account entirely.
My friend also thinks that this problem could have stemmed from making an Xbox account directly before making a Microsoft account.