Does watching videos that are on the camera roll use cellular data? Someone asked and I said I didn't think so. Battery, yes.
If they're streaming from the iCloud Photo Library, then, yes if not on WiFi.
I shoot a 20 minute video and it's on my camera roll, but not in the iCloud Photo Library. If I'm off wifi, does it use data? Local music does not.
He took a bunch of concert videos and they're all on his iCloud Photo Library. He wants to know how he can view them without using cellular data. I don't see a good way for him to do that.
He'd have to transfer them to the phone while on wifi or via iTunes, and then he can watch them on the phone.
If they're not stored on the phone and he's on a cellular connection, then that connection is the only way to get the data to the phone when he wants to watch them.