Ethan Leib has published an article called Friends as Fiduciaries, 86 Wash. U. L. Rev. 665 (2009). Here is the abstract:
This Article argues that the law of fiduciary duties provides a good framework for friends to understand their duties to one another better, gives courts a useful set of rhetorical and analytical tools to employ when they are forced to entertain disputes that arise between close friends, and, finally, can help direct courts to furnish betrayed friends certain kinds of
remedies that are most appropriate for achieving justice within that dispute context. This is not the first Article to make an effort to expand the reach of the fiduciary concept into new sorts of relationships that are not always considered within the ambit of fiduciary duty law. But the case for thinking of friends as fiduciaries is exceedingly persuasive and underappreciated, both in the law and in our lives.
Here is the link to the pdf: http://lawreview.wustl.edu/inprint/86/3/leib.pdf