I was looking for a different kind of Sunday brunch a few weeks ago, JC, and stumbled upon this place : http://newyork.5thavenue.andaz.hyatt.... [the entrance is on 41st a dozen steps east of Fifth].
Andaz is a Hyatt boutique hotel, so it doesn't have an industrial feel; the food (breakfast/brunch) was a cut above the standard gluten-gala; the service was attentive but not intrusive; the space -with the façade of the main NYPL showing magnificently through the picture windows- is cinematic-Manhattan-special; and (with all due respect to moto and elizabeth) actually near to where you'll be: less than a five-minute walk from Park & 39th.
In sum, it's a distinctive & charming place that your client might well appreciate.
Penelope on 30th and Lex has excellent breakfast
If diner fare fits the bill there is Scotty's Diner on Lex and 39th
L'Express on Park and 20th is good too
249 Park Ave S, New York, NY 10003
336 Lexington Ave, New York, NY 10016
130 E 39th St, New York, NY 10016