If someone borrows something and it gets stolen from them who is responsible by law? Negligence, Other Injury

The gears on my bike were not working properly so I took it to a friend and asked if they could repair it. They fixed and adjusted the gears on my bike so that they worked properly. They wanted to try the bike out on a ride with some friends. I told them that this was acceptable. The bike was stolen off their third floor apartment balcony where the bike was unlocked. What are the principles of law that apply and who is responsible?