That study was about VCs choosing investments, not startup founders working long, stressful hours side-by-side. I realize there are disadvantages to working with friends, but I'm pretty sure the advantages outweigh them. Paul Graham seems to agree, and he makes a living picking founders.

http://www.overcomingbias.com/2012/06/good-friends-can-make-bad-business-partners.html

I agree and add my own personal experience as an anecdote. Business gives different incentives and prompts different applications of power. I no longer have several friends, for most part due to business related problems.