To fly in the USA you will need to either have an FAA licence ('certificate' here) issued on the back of your foreign licence ie the FAA cert. is only valid while the foreign one is valid *or* sit the FAA theory & flight test for the issue of an FAA certificate.
Your friend will need to sit the FAA ATP theory exam and a flight test.
You and your friend will have to satisfy the various security stupidities that are now required. Do a search on these forums & you'll find plenty of threads about what you need to do. For the FAA certificates the applicant will have to meet FAA minimum experience requirements.
Note that the FAA treat each class & category of aircraft as a separate certificate eg Single Engine Land, Multi Engine Land, Single Engine Sea, Multi Engine Sea, Rotorcraft *and* and each licence level as separate certificate ie PPL, CPL, ATP.
This leads to the circumstance that you can have privileges for one licence level valid only in a particular class/category, and privileges for another licence level valid only in a different category eg you could easily hold a PPL-SEL, CPL-SES and ATP-MEL. In my case the only FAA certificates I hold are at the ATP level because those are the flight tests I chose to do when I converted to the US certificates. The three I hold are ATP-MEL, ATP-SEL, ATP-SES.
An advantage of the FAA ATP is that it has integral instrument rating privileges built into the certificate so no need to do a separate instrument rating exam & test.