I think both had a hand in it
Written by LouAnn
(10/19/2009 11:49 a.m.)
in consequence of the missive, Interesting question!, penned by Louise H
I think Sophia was involved, even if the letter is not all her words, because:
she found the letters and the hair and wanted to be sure he returned them;
she wanted to be sure that Mrs. Jennings or any other gossips saw the proof in writing that Willoughby was 'otherwise engaged' and that he and Marianne were 'just friends' even though Marianne had written to him.
I think a letter written by Willoughby alone would have had a different tone.