Jocasta is Vyvyan Ayrs's wife (she kept her own name; not enough vowels in his, we guess...). She is not only having an affair with young Frobisher, but she's also rumored to have had "a long line of youthful stallions invited to forage in her manger" (2.5.5). Although she appears often in Frobisher's letters, they're always about the same thing. She and Frobisher sleep together, she wants to talk about her feelings, Frobisher doesn't want to listen, lather, rinse, repeat.
If we didn't already know Frobisher could be a callous, selfish cad, this seals the deal. However, when she accuses him of using her he says "of course I've 'used' her; just as she's 'used' me too" (2.9.3). He made it clear early on that the relationship was purely physical; she's the one who wants something else out of it.
It's also really weird, by the way, that this lady's name is Jocasta. Pretty much no one has been named Jocasta since that named belonged to that queen-mother of Freudian nightmare in Sophocles's Oedipus the King. The relationship in Cloud Atlas is pretty incestuous: Vyvyan Ayrs is a father figure for Frobisher, so Frobisher's affair with Jocasta is nasty on more than one level. Maybe that's why he turns out almost as badly as Oedipus does?
Well, it becomes clear in the end (if we can trust Ayrs) that Jocasta was all part of the plot to keep Frobisher in Zedelghem long enough for Ayrs to capitalize on his talents. With her as wing-woman, Ayrs can treat Frobisher however he wants, and Jocasta can pick up the pieces and put them back together. Who's using whom now?