What I have done in similar circumstances and recommend is to get a room at the airport -- say the Ibis or Novotel which sits atop the RER station -- and then take the train into town for dinner, an evening walk along the Seine -- perhaps even a Seine boat cruise. There is plenty of time to walk around Notre Dame, and the central islands, to have a cruise and a good dinner. With the cruise he would also see the Eiffel Tower twinkling at night with little effort.
He would return via RER late that evening and be able to get a good night's sleep and easily be at check in for the flight the next morning.
He could certainly to into Paris for the night, but personally I don't see getting much out of the morning since he needs to be at the airport 3 hours before a flight to the US. But an evening in Paris would be delightful and getting up at 7 instead of 5:30 or 6 the next morning would be a winner for me.