Currently he is working with Company A.
* His sponsor is Company B and he got I-140 approved last
week.
If his I-140 is just approved last week then how did he cross 180 days after I-485? Do you mean that it was concurrent filing? If that is the case then the 180 days should be counted for I-485 after approval of I-140
To use AC21 (AC21 the law was passed earlier than concurrent filing started AC21 has nothing for concurrent filed I-485) But lawyers interpret that if after I-140 approval then I-485 is pending more than 180 days AC21 may be used
* He is trying to join with new Employer C, after receiving EAD.
-- Better he should cross 180 days, if he wants to change immediately after EAD then company B should not revoke his I-140 and he should find same/similar job and should get wages according to approved LC
It means, he will never work for company B, who is his
sponsor.
-- No, problem, GC is for future job and he can use AC21, new employer should be able to give him permanent job offer letter and lawyer can send AC21 latter. He should clear everything with new employer before he joins him. He may also request new employer to file for H1 transfer and join on H1 status, because if due to any reason his I-485 gets denied he will have other status to stay and work in US.
Will it be a problem, in case if he faces I-485 interview?
--- Every case is different, it cannot be generalized, depend upon his past status and what will be in the final AC21 regulations when they are published. He should speak to good immigration lawyer and follow his/her advice