Get over it. It doesn't hurt that bad. It's not even bruised!
Run after them and give them a peice of you're mind.
Let them get over it by themselves. It doesn't involve you.
Pick sides. You're her/his friend, and he/her is right!
Act happy for you're friend.
Turn against my best friend.
Force them to pay you back. How could they betray your trust?
Tell them it's okay, and be careful about what you let them borrow in the future.
Scream and freak out.
Keep doing whatever you're doing.
Run away, into you're room, and cry. You have such nice neighbors!
Think about the idea before reacting. It could be fun!