Upload a video from iPhone to someone else's YouTube channel?
She's most likely out of iCloud storage space. The edited video is likely to be beyond the allowed limit of email attachment. Her iPhone is attempting to upload the video on iCloud, and send a link to the same.

The current iCloud storage usage can be checked by going to Settings app → Her name (at the top) → iCloud → Manage Storage.

If she's out of storage space on iCloud, you can advise her to use any of the alternate methods as mentioned below:


If both of you are in close proximity and you use a Mac, you can simply ask her to AirDrop it to you.
Ask her to transfer it to a computer at her end and send you the video using any alternate file sharing service such as Dropbox or Google Drive.
If she doesn't have a computer handy, she can install the client app for any of the file sharing service as mentioned above (Dropbox, Google Drive), upload the app right from your iPhone, generate a link and send it to you.