Of course, ethics isn’t maths. There isn’t a right answer you can work out or look up. Different ethical approaches may give different assessments of ethical questions.
My undrstanding is pretty limited, but I belive there are three main strands of ethical approach:
Virtue ethics: one should aim to think, feel and act in a morally good way with the ultimate aim not of doing good but of being good, of embodying virtue. Relevant virtues here might be honesty and trustworthiness; equally they could be thrift and loyalty to one’s family above all. The question is: is Joe being a virtuous person? Is he being good?
Deontology: one should adhere to moral obligations. This is Kant’s turf, and also where I get very fuzzy. But basically, there is a set of principles to judge an act against - if it follows them, it is ethical. If it does not, it ain’t. Only act in a way that you would be happy if it were to be the general rule. One principle that we might be happy to have as a general rule is: don’t break the terms of a deal. Another might be: everyone should be free to get whatever they can get away with. Here the question is: is Joe being true to his principles, and acting in a way he would be cool with if everyone else did it too?
Consequentialism: all that matters is outcomes. If what you do ends up with bad things happening, then it’s bad. If good things happen, then it was a good thing to do. This raises the question of how well an actor can judge the consequences. In fact, this is the question: do Joe’s actions hurt the car wash, are they neutral, or as @chela hypothesised, might they even benefit it? If Joe has good reason (discounting his self-interest) to believe he knows the answer to this question, then he knows if his actions are ethical.
How you choose which framework to use is a puzzler as strictly you can’t do it on the basis of an ethical judgement, as you haven’t yet got an ethical framework from which to make judgements!
But it’s interesting, I think, that most judgements that Joe is unethical stem from a virtue or deontological framework, while most judgements that Joe is ethically neutral or perhaps an unsung hero stem from consequentialism.