We don't know what Emma says exactly. JA says that She [Emma] had to ...acknowledge herself grossly mistaken and mis-judging in all her ideas on one subject, all her observations, all her convictions, all her prophesies for the last six weeks. , which presumably Emma did do. And JA does say that Hartfield was where Harriet could have any chance of hearing him spoken of with cooling moderation or repellant truth.

The difficulty apparently is in persuading Harriet of the repellent truth. When a person feels they are in love, they have a tendency to not believe bad things about the loved one. I can easily imagine that Emma would have felt reluctant to repeat to Harriet the most damming words out of Mr Elton's mouth about him being above Harriet, because said words would hurt Harriet directly, and everything else Harriet could explain away.