erm, i don't really know what i'd do... for sure I'd be disappointed in that friend, but no one is perfect i guess... especially not me...
judging from my pasts, when my so-called friends turned their backs on me, (and i do assume they were talking in my back) i just respected their choice and went away... after that, i remember me hiding in the girl's bathroom in between classes back when i was in high school coz' i was ashamed of being alone and not having anyone to talk to, but then i met other people, people that i still consider very good friends ^^... and about those people who turned their backs to me, i kept my distances from them for a while, coz' they made me feel bad and i was mad... but after a year or two i started talking to them again, just casual conversations but, it's not like i have any anger or bad feelings towards them anymore...