(not a lawyer) This depends on factors not mentioned in the OP.
Basically, to win, John has to show that Sally made a statement of fact (not an opinion - “Seems to me he’s acting like an embezzler”), that implies not disclosed facts (If Sally says “I’ve see the company’s books; John’s an embezzler” she’s in a lot worse trouble than if she says “Based on what the papers say, John’s an embezzler”), which is not true (if Sally has a copy of the company’s books, and they show fishy numbers, she may be okay - even if Jim, John’s partner, turns out to be the real crook), and which degraded John’s reputation (if his reputation already is awful, or no one believes Sally at all, Sally’s in the clear). So Sally has a bunch of ways to not be liable – truth being one of those ways. There are lots of other factors too, I think - but remember, I’m not a lawyer.
https://www.washingtonpost.com/news/volokh-conspiracy/wp/2014/05/16/libel-law-and-inferences-from-disclosed-facts/ and A Little Libel | Whatever have some interesting insights (and the first of these is written by a lawyer).