I would ask your friend whats wrong with it. It sounds like they might have sold you a phone that is damaged. Best case scenario is that it's not damaged and the battery just needs to be replaced, which you can get a new one put in at the apple store pretty cheap and it would be like a new phone again. Thats of course assuming someone didn't sell you something thats damaged.
How did you look at it before buying? I assume you checked it out first and seen it "on"??