Thanks for the replies.
A factor of six for fraudulent hour building does seem a bit implausible. OC 619's suggestion of a low-revving engine in a boat would seem more likely, but for the fact, which I didn't mention, that the seller was also selling the other instruments from a light aircraft panel, makes this less likely. From the description in the adverts for the tacho and his other instruments, I suspect the seller did not know a great deal about what he was selling.
My friend isn't unduly worried as he now has parts to either fix the original tacho, or get this one working again properly.
Step Turn, I appreciate your concern, however the owners do understand what can and can't be done within the regulations for this aircraft.