She's had an issue with going over a broadband monthly data cap (on copper). Mostly Netflix on her TV with a chromecast. So, she asked to move to an unlimited plan, and was told sorry, your router can't do unlimited. What? Can anyone explain how that might be so?
So, they sold her an unlimited mobile plan for her phone; no tethering allowed, but she "can do Netflix by mirroring from her phone". What? Can anyone explain how she could mirror to the chromecast while using mobile internet? The broadband connection is still running, for other uses.
Now, in principle, the wifi router could be disconnected from the internet, and the android phone set (via developer options) to keep the mobile connection going while on wifi, and so serve internet to the chromecast, but I'm very sceptical about that. Or does the Netflix app have some magic she hasn't found; if the phone is on the wifi, mobile data is not used, and if not, the cast button is not there.