Need Rec's for mother's birthday dinner- 20 mile radius of Rhinebeck NY
This is stretching your mileage limit, but across the river in Milton on Rte 9W lies The Ship Lantern Inn. Classic continental cuisine, tuxedo-ed waiters, great food, superb service, lovely romantic dining room. It's a total throwback place (est 1925!), but there's nothing musty or dowdy about it. You'll have a memorable experience and your mom will be wow-ed.
The Beekman Arms has atmosphere, but the food is forgettable. I like Starr Place in Rhinebeck, but if you travel only a short distance you can go to Madalin's Table in Tivoli, which is a bit more formal, very charming, and has excellent food. No reservations taken, but there is rarely more than a few minutes wait.
I wouldn't recommend "the Beek" for dinner. There are a lot of good restaurants in Rhinebeck, you'll see Terrapin recommended on this board.
If mom is a local then how about Foster's? Not fancy but she's bound to see people she knows and might enjoy the opportunity to show you off!
Across the river Reginato Ristorante in Lake Katrine has an old world charm that appeals to members of the senior set.