WireNuts, how hot was that hot water pipe?? It would take a steam line to melt through the 90C-rated insulation in NM-B, but most people don't even set their hot water heaters above 60C. And even if the romex did melt through and short out to the pipe, that *should* solidly ground it to the GEC and short-circuit trip the breaker.
Plumber often wrap BX to prevent electrolosis but romex is no problem as it is insulated. excellencee's HO/plumber was thinking bx but he was a plumber and what does a plumber know about electrical installations ;-)
BX isn't going to cause electrolysis, either, though. I think some plumbers are just paranoid