Much easier than the other way round!
As long as your friend fulfills the FAA ATPL flight prerequisites then he is allowed to go straight to the FAA ATPL.
Holding an ICAO CPL/ATPL with full Instrument rating he will need to complete:
1. FAA ATPL Written exam
2. Complete an FAA oral and flight test.
Most only complete in the twin.
Happy New Year (almost)
Nikki