My first question is whether your client has to work/interact with this person regularly (coworker?) or is it someone they can avoid? Second, is there some inherent reward to the revenge rumination (fantasy/satisfaction)? Otherwise I have heard of a few books mentioned in the past, although I have not read them. The first on is “Dealing with People You Can’t Stand”, a self-help book but more for the motivated. The other is “People Can’t Drive You Crazy If You Don’t Give Them the Keys”, told there was a Christian undertone to it (again, have not read them).
IF you have a really good rapport, maybe try confronting it? While doing family therapy, I once used a “speak and repeat” toy to replay a father’s words back to him when he engaged in juvenile revenge fantasies (high squeaky voice from a fuzzy hamster). He got a good laugh (client had a sarcastic sense of humor), said it sounded like something his child would say, and was able to recognize how irrational his comments sounded/poor role model for child – but definitely not for everyone.