Does anything in FAA land or the RFM specify the need for an artificial horizon or attitude indicator to be fitted for night flight in a Robbie?
Night VFR is a misnomer, especially over poorly lit ground or sea. You might be able to see other aircraft but probably can't see any ground definition and the visual horizon can be very difficult to define.
Nope, but you are required to be able to see the ground, either by lights on the ground, or adequate celestial illumination.
Most R44's I've flown though, have had one installed, and (to my surprise) the last R22 I flew even had a glass one, with a glass HSI below it.