If he's doing video recording, I would think most digital camcorders have video-in... kill two birds with one stone.
If you want a standalone device, the only thing I can think of offhand is a portable DVD player. Most of those also have a video-in, and are pretty small (and battery-powered). Bigger than a credit card, though, by a long shot. But relatively cheap.
You might be able to mod a PocketPC chassis...but I don't know how you'd interface a 'normal' video feed with the LCD display on those things. You'd need to rip the electronics out of something that had a video-in and used a compatible LCD panel, then hook it up to the PocketPC's display and battery (which would probably run down REALLY fast). Not a project for the faint of heart.