It's not difficult and not too time-consuming. I'm assuming the checked bags will be routed from ICN-PEK-EWR without the need for your relative to ever see them until EWR. The procedure will be:
--deplane with all other passengers, ride the shuttle train to the main terminal area (unless if coming into C), and move to the immigration desks.
--relative should look for the desk marked "international transfer" which at T3 is often by itself, just before the long line of regular immigration desks. The officer will need to see passport and onward boarding pass (also have the first boarding pass handy in case officer wants to see). This does not require a valid Chinese visa as your relative will technically not be entering China.
--then relative goes back to transit side and to next gate, which will almost certainly involve a shuttle train ride back to the airside building E. Things are pretty well signed.

I could make this change in elapsed time from exit jetway of first flight to gate of second flight, usually in 30-ish minutes, and flights to USA will likely start loading at 45-50 minutes before takeoff. But, it would be better to have a 2-3 hour scheduled layover just to make things more leisurely.