How can I cast a video file that has been downloaded onto an iPad to a chromecast
She can use the Files app on the iPad to copy the files into Photos (what you call the camera roll). Simply open Files, navigate to the &quot;my files&quot; directory, tap on the video and press the Share icon (square with arrow pointing upwards). In the Share sheet she should select &quot;Save video&quot;. This stores a copy of the video in Photos.
A different approach is to install a video player app such as &quot;VLC&quot; from the App Store. Now in the Files app, open the share sheet for the video and select to share it to VLC. Now it will play in VLC and you can tap the Chromecast icon to get it on the TV.