The guy who led the research was on Radio5 tonite, worth catching on the Listen Again feature because he corrected a lot of the way the story is being misreported.
He said that if your friend puts on 40 lbs, you tend to be more likely to let yourself go. And if your friend lost 40 lbs, you felt a social pressure to do similar (to losing weight is contagious, too). It was friends changing their weight that had an influence, not whether they were fat or thin already.
Oh, and if your good friend with fluctuating weight lived 2000 miles away or next door had equal influence, as long as they were equally important people to you.