I don't know if you / your client are new to Paris, or if you already know where you will have diner. But my favourite when I only have a few hours (in the evening) with a "first time" visitor is:
- RER to Etoille (connecting in Châtelet) to view the Arc de Triomphe and a quick walk on the Champs-Elysées.
- Short metro ride to Trocadero to view the Tour Eiffel from the viewing platform (across the Seine). If find the view from there more impressive than from beneath the tower.
- If you have time, add a stop at Châtelet on the way back and walk to the Ile-de-France and Notre-Dame. (You can take the RER back to CDG from St.-Michel/Notre-Dame.)