A friend had his bike stolen. He had a heavy cable protecting it. Are all locks/cables subject to cutting off if the cutter is heavy duty? So no bike is ever really safe then?

Nothing is safe from a determined criminal. The best protection is to give the criminal something else to think about, i.e. your lock is stronger than the other locks on bikes. Your house has a security system when others in the neighborhood do not.

thats mean that someone stole it like that

Thank you for your answer Nevan. A U-lock? Okey dokey! :)