Schiphol has a main arrival/departure hall, with the train station/tracks underneath, so this would be the most logical place.
Outside the secure zone there, is Starbucks, a Grand Cafe Salon, and a Koffiesalon. All good places to meet up, and for you to wait.
You could also wait in the secure zone, Amsterdam is a bit different in that when you exit the plane, you just walk out into the international terminal, (assuming you both are arriving from the US) with shops and restaurants, then immigration is later as you leave. So you could sit, have a coffee, and then meet your friend as they walk off the plane.