Hi again all,
Firstly - a big thank you to everyone who responded via PM or on the thread with offers of assistance. The broader aviation community really are a helpful bunch, thank you!
The issue is indeed that CASA will only consider an ICAO compliant Level 6 certification for conversion, and the FAA license is different.
As a result of the assistance we received, we have managed to find a solution for my friend to sit the test in the USA.
all the best,
Fower