Weird! It sounds like, at some point, the camera fried the SD card; otherwise, there at least would be a format on it (so it'd be read by any pc (Mac or Win). As for avoiding that from happening, the only one I know is to always set the safety latch on the card after video-recording.
As a last resort, I would try to read that card using Disk Utility on a Mac. And, if it mounts, apply repair.
Good luck to your assistant!
Alberto