training by a sub-Icao IR instructor doesn't count as dual training towards an FAA IR
This is half the problem here - this assumption is not always right, and therefore an allegation that somebody else has a dodgy license is not likely to be correct.
And doing one's IMCR with an IR holding instructor meets the strictest possible interpretation of the FARs re "authorised instuctor" and the other stuff (haven't got the FAR/AIM to hand) authorising training outside the USA.