Setting aside the ethical question for a moment (but only a brief one), the kit still
legally belongs to the guy who left it there. Just because you can justify in your own mind that a) he left his drums behind and b) he owes your friend money and c) the drums could be confiscated in exchange for the money, doesn't make that legally right. The law is clear in such situations that you may NOT take someone's property in exchange for money that person owes you, without that person's permission.
However, if you want to acquire the drum kit, you must go through a process of having them legally declared "abandoned" property. In order to do that, at the very least, you'll have to document that you've made a thorough effort to contact the person and also a significant amount of time has to pass so that he can spontaneously return to retrieve his belongings. The requirements vary from state to state and attempting to just improvise this without consulting competent legal advice on how to proceed will make you a CRIMINAL if the person returns and finds that you've illegally taken his stuff for yourself.
And it's unethical.
