J M Barrie and Florence Hardy were engaged in making significant additions and deletions to 'The Early Life of Thomas Hardy, 1840-1891' and 'Later Years of Thomas Hardy, 1892-1928', which had been written by Thomas Hardy, and which were published in 1928 and 1930 under Florence Emily Hardy's name (and later discredited). But I am not aware that they were engaged to each other.
One incident in which Barrie caused no small upset to Florence Hardy was on the occasion of the unveiling of Hardy's monument in Dorchester in 1931. In his speech Barrie started by saying that at his birth Hardy was such a weakling that the nurse had put him in the wasking-basket for dead and directed her attention to the mother instead. Florence was outraged at this and later told Barrie that T.H. had never mentioned the incident: "I asked Barrie to tell me about it when we returned to Max Gate. I said that I had never heard the story. Then he remarked: 'Well, I put that bit in. It was a good piece of drama. So I put it in!'" "That," Florence responded, "is how legends begin. Like Wellington's 'Up Guards and at 'em' at Waterloo. I think it monstrous."