Well, first step would be a validation of his PPL, resulting in a US PPL based on the foreign one and only valid with both present. This can be done at an FSDO (Flight Standard District Office) for free. It can be done by mail as well. Also many flight schools experienced with foreign students can arrange it at a fee (usually in the 50 USD range). Going through a flight school may be the fastest way - for a CPL he needs one anyway. (Reference)
A foreign CPL (and everything else but a PPL, so like an FI) cannot be converted or validated. He will have to go through full process as described in 14 CFR 61.123. This needs signing up to a flight school/instructor. So while his hours will be accounted for, he need to do at least an extensive check ride (usually more than one - as procedures may differ) to convince the instructor to be ready for the practical test, as well as going through all knowledge tests required - and fulfil the language requirement as well.
A (good) school may also be the best place to get more in depth information, as this case may hold many individual details to be considered.
I guess the worst part might be to (re-) learn the standard phrases used in the US vs. what is used in his country of origin. 8000 hours means a lot of experience and deep entrenched radio phrases :))