Does anyone know if a JAA licence holder can legally hour build in the US or do they need to convert their licence to an FAA one?
You DON'T need to convert. If you're a licensed helicopter pilot hours flown are hours flown.
BUT, hours don't count for a type rating below 500 on type, and to SFH you need to get an FAA licence issued on the basis of your JAA licence at the local flight safety office and post-911 there are a whole load of security checks to be carried out first.
AND If the hours are supposed to be part of a JAR course and aren't, guess what? They don't count
AND As Whirlygig said, FAA & JAA definitions of loggable PIC differ. Under JAR only one pilot can log PIC in an SP helicopter. So that would be the higher-time Boatix CFI unless you are specifically nominated as arcraft commander and have responsibility for the flight.
TT