@Gdmixon
NetFlix does not use the AirPlay casting/mirroring protocol - it uses a casting protocol called DIAL which cannot be disabled/controlled directly.
As long as the Roku & iPad devices are connected to the same network/internet connection she'll be able to cast NF (using the cast icon from within the NF app).
You have a couple options:
1) Train/teach her not to use the cast icon within NF or
2) Create a Guest WiFi SSID within your modem/router/gateway's settings and assign/connect the iPad to the Guest WiFi SSID (guest SSIDs are almost always limited to only accessing other guest SSID devices/blocked from non-guest SSID device access). Note: not all modem/router/gateway's allow Guest SSIDs, though they may otherwise have a setting/function called "AP isolation" which achieves the same thing but may/will impact all devices on your network.
