Sunday Brunch:Theater District?
Marseille is at 44th and 9th - and gets a very heavy pre-theatre crowd (and deservedly - it's a good place with interesting food at reasonable prices). In the same area, we like the West Bank Cafe (42nd west of 9th - contemporary American cuisine), which serves a nice Sunday brunch, and is calmer/quieter than Marseille. Depending on how high up in the 40's they are, the OP's friends might consider Taboon at 10th and 52nd; I haven't had brunch there (just dinners), but their pan-mediterranean/north african dishes are very good (and their house bread is great) .
That said, everything in the west 40's is "close to the theatre district", although restaurants on 9th and west of 9th tend to get more locals and fewer tourists. The OP's friends will still face a hefty Sunday matinee crowd, though, which is pretty unavoidable in that part of the city.
West Bank Cafe
407 West 42nd Street, New York, NY 10036
773 10th Avenue, New York, NY 10019