How is the coverage different? The flash covers x degrees, the lens is y degrees. It is field of view dependant, not format size.
I find my 30D and Gossen Luna Pro agree on reflected just fine.
dSLR will show you what the image looks like just fine - where the light falls, the softness/hardness of the light, how the lighting contrast looks.
Flash meter is a great tool too.
I use a pair of YN-460 II'swith my RB67 and RF-600/602's.
Assuming he doesn't have a dSLR already - or any small mirrorless camera with hot shoe or pc sync. There are a lot of cheap small p&s with hot shoe.
Great for preview when you're doing things on the cheap and not having modelling lamps on your flashes.
Even with a digital preview or polaroid back - neither is a substitute for an incident flash meter, that should be first priority.
Agree. This absolute insistence on "analog only" is approaching "flat earth" looniness when it comes to issues like this. I haven't seen anyone use Polaroid proofing in a "film" studio session in years.