A) It's not your bike, so the OP "needs" to do nothing. While not as effective as a front brake, you can stop a fixed gear without one.
Hehe, until his chain breaks or comes loose, or he is skidding and runs into me (true story).
I apologize if he was going to put a brake on it, but brakeless fixed gears with dumbass riders have crashed into me, and I was on my bike. They crashed into me because they couldn't stop at a stop sign that I had the right of way on and was riding through. So the argument that brakeless riders are only endangering themselves is BS.
Riding brakeless is stupid, and I am going to deride anyone who does it.
Fixies are awesome, as long as you run at at least a front brake.