Shinagawa is easier to navigate and has (I think) just one Starbucks, so that would be an obvious place, but if you must meet at Tokyo Station, Daimaru is pretty unmistakable, on the Yaesu side of the station near the Shinkansen ticket counter. As I recall from this summer's trip, once you have your ticket, there is no obvious place other than the platforms to sit down and have a snack or drink; for that, you have to go to the underground level, which is a labyrinth of stores and restaurants. But Daimaru is easy to identify and easy to navigate.