Why would your friend hand the ticket for a different flight to the gate agent in the first place? Or did he just hand over the whole stack of tickets at the gate and rely on the gate agent to pull only the correct one? I'm just trying to understand how he believes the gate agent pulled the wrong one.

I'm sorry your friend had problems. I think he may be entitled to a refund of the money he spent over the $100, if it can be shown that he should have only been charged that for the lost ticket. However, beyond that, I doubt any further compensation is available.