Casey changed her Apple ID password because she thought she was getting hacked and people were using her data to watch YouTube. Apple said that she wasn't, though she says she hasn't been using her data and it's almost entirely used up for the month. Jason says that her Apple ID shouldn't have an impact on YouTube, so he's skeptical about whether it's connected to her Apple ID. More likely it's preferences for YouTube.
Casey should go into the Settings app, and under cellular she can scroll down and see what data is being used by what app. While there, she can turn off Wi-Fi Assist, because what that does is use Data when the Wi-Fi is bad.