I would not use nitro in a BP revolver, or any BP gun.
I don't think the steel is good enough to stand the higher pressures. I suspect that many reproductions use the minimum grade steel and are case hardened to reduce wear.
The quality of the steel on originals, locks, triggers etc. is harder than modern reproductions. And the steel on some original barrels seems better too.

I have not tested the hardness of the metal but how a file or oil stone cuts the surface is a fairly good guide.

The priming compound in a shotgun primer is probably 2-3 times more than an ordinary N0.11 cap. That extra brissance from the shotgun primer will help to increase the pressures with the nitro. It is like using a magnum primer!

He should try the shotgun primers with BP and work a load up.