Astrid is one of Moushumi's best friends from Brown, and is married to Donald, an artist. They're the sophisticated, artsy, cultured couple that introduces Moushumi to Graham. Moushumi envies Astrid and Donald's lifestyle, which is full of art and culture, and her own marriage to Gogol compares unfavorably to theirs.
Astrid doesn't help matters, either. She tells Moushumi that she can't see her with an Indian guy, as if marrying an Indian man were somehow degrading. For his part, Donald does his fair share of mucking things up for our young Bengali newlyweds, too. He tells Gogol that Moushumi and Graham were "too similar" and "too intense," a subtle way of saying that they loved each other in a way that Moushumi does not love Gogol.
All in all, these two are not very round characters, but they certainly help illuminate some of the problems in Moushumi and Gogol's marriage. Moushumi wants what Donald and Astrid have, and Gogol can't give it to her. It's about as simple as that.