You don't say from which version of OS X he upgraded, but is his current iMovie the latest version?
I suspect that the problem lies in his user account, since iMovie is not incompatible with Yosemite; I just opened my version.
How about logging into another user account (or create one) and see if they can be retrieved from iMovie in this test account.
On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.14.x,; iPhones, iPods and iPads galore!