If you're able to install software on the device, GlassWire would work. If not, you're going to be looking at Netflow to capture the data and something like Solarwind to make sense of it. Most consumer grade routers won't support Netflow however, and it'd be more hassle than it's worth setting up you network on an entirely new router just for this, unless you know a network engineer who's willing to set it up for you. I'm using a Cisco 2620XM that supports this, but it's only really suitable because I'm a sad network engineer who gets excited by configuring networks and I only have a slow old DSL connection that doesn't stress the older hardware in this device.
The other option would be to ban her from using it for a week and see how much the difference in your data usage is for that week. Most routers will offer options for blocking based on MAC addresses. Just be warned that some devices allow you to change their MAC addresses. You will need to search for how to filter mac addresses for your specific router, and also how to find the mac address for you specific device. On windows this is done through "ipconfig /all" after launching "cmd" from your start bar.
