Good heavens! My first impulse was to think that this is some form of redirected aggression, but I think a behaviorist is the best choice in this case. Usually cats can exhibit different kinds of aggression:
Play aggression, displacement(redirected aggression) or overstimulation. However, this situation seems to be different. How long has your friend had Sam? Is this something he has always done, or did it start recently. Since he is a rescue, it's hard to know what his background is. It could be a psychological problem or possibly a physical problem. An animal behaviorist should be familiar with these different kinds of problems and what kind of behavior a cat may exhibit (fear, anxiety, redirected aggresstion...etc).

If a cause can be identified, either physical or psychological it is easier to predict whether or not treatment will be beneficial and obviously easier to choose the treatment plan most likely to work.

Good luck and do let us know what happens!!