It's the videos. Both you and she should be on WiFi when streaming whenever possible. Where is she living? If it is a dorm or apartment and she doesn't have good internet that should be fixed. Likewise your iPad (and everyone in your house) should be going through your home's WiFi when looking at YouTube videos or streaming shows. Make sure she knows what her phone is connecting through. It should be a WiFi outside the device, going through your internet plan, not the phone (or iPad's) own internal "hotspot".
You might be able to disable the hotspot feature through the phone's plan, I know I pay extra for the ability to use my phone as a hotspot. I use it when staying at hotels that have slow internet connections, especially in the evening when a lot of guests are using it at once. I also use it if I access bank accounts while I am at hotels or other public places that do not have security encryption on their WiFis. When I visited Mom and she had no internet at her house, I used it to stream Netflix and I'd go over my data, but it was worth the $15 or $30 once or twice a year, rather than increase the data plan and pay more each month.
Under no circumstances accept slower speed.
As for dealing with her, you're going to have to sit down with her and go over how her devices are connecting and make sure she understands the cost consequences. The "hotspot" setting needs to be OFF most all the time. It's possible she is falling asleep watching shows and they're running for hours (I do that!) If she is binge watching, that can be a problem, akin to gaming addiction or other massive wastes of time. She might also be texting a lot; there are ways to set your phone to use WiFi instead of data for some text messages. Also go into all the applications settings on the phone and turn off the ones she isn't using, and turn off WiFi assist. (WiFi assist means the phone, if on WiFi, will bump over to data if it thinks the WiFi connection is flaky.)
I don't expect she will have any interest in learning all these things though unless you make her personally responsible for the data cost which won't happen until you kick her off your plan and make her get her own. For us, the time was right when they got boyfriends. Then they wanted to get on plans with their guy, and so another piece of the umbilical cord is cut. Bittersweet, that. Enjoy her needing you while it lasts.
If she knows she is using the data for streaming because of slow internet performance, yet upgrading the internet will cost money, you pay one way or the other for more/faster data, that's just reality and if you want to maximize the use of technology, we have to bite the bullet. Personally I won't settle for less than 50 Mbps download these days. If you don't have that already in your area, your ISP should be working on it; I'd say something if they aren't.
