Emma's behaviour in Chapter 26 I find hard to forgive. It is bad enough that she has formed an unpleasant fantasy about Jane Fairfax and Mr Dixon, on very flimsy evidence - we, of course, have the advantage of knowing that it must be a fantasy since the narrator told us back in Chapter 20 that Miss Campbell engaged the affections of Mr. Dixon almost as soon as they were acquainted. It is almost as if Emma was looking for something bad to think about Jane. And now she decides to share her nasty beliefs with a man she has known for less than a week. The fact that Frank Churchill does not politely stop this line of conversation (he could simply have said "Miss Woodhouse, I cannot agree with you and I think we should change the subject.") at once speaks very ill of his character too, but two wrongs do not make a right and the fact that he encourages her to develop her theme does not excuse Emma for ever having raised the subject.
And she knows it herself - in Chapter 27 when she reflects on the previous evening "She doubted whether she had not transgressed the duty of woman by woman, in betraying her suspicions of Jane Fairfax's feelings to Frank Churchill. It was hardly right; but it had been so strong an idea, that it would escape her, and his submission to all that she told, was a compliment to her penetration which made it difficult for her to be quite certain that she ought to have held her tongue." Can there be any readers who are not quite certain that she definitely should have done that? Had John Knightley only been present and overheard her, he might have given her some legal advice on the subject of slander.