The short answer is yes, conduct that occurs off duty can be charged as maltreatment, because the offense under Article 93 of the Uniform Code of Military Justice turns on the relationship between the people involved rather than on the clock or the location. Article 93 prohibits cruelty toward, oppression of, or maltreatment of any person subject to the orders of the accused. Nothing in that definition limits it to the duty day. This article explains why off-duty conduct can qualify, what the government must prove, and where the limits lie.
The offense centers on a relationship, not a time of day
Article 93 has two core elements. First, that a certain person was subject to the orders of the accused. Second, that the accused was cruel toward, or oppressed, or maltreated that person. The phrase that does the work is subject to the orders of the accused. The victim must be someone the accused had authority over, such as a subordinate, a trainee, a recruit, or another junior member in a relationship of authority.
Because the offense is defined by this superior-subordinate authority relationship, the conduct does not have to happen during scheduled duty hours or on a military installation to be covered. A noncommissioned officer who harasses or abuses a junior member during off-duty hours can still be reached by Article 93 if the authority relationship is the reason the conduct has its coercive force. The supervisory relationship does not switch off at the end of the workday.
What counts as cruelty, oppression, or maltreatment
The conduct prohibited by Article 93 is real abuse of authority, not ordinary unpleasantness. Maltreatment is measured by an objective standard, meaning the question is whether the treatment, viewed reasonably under all the circumstances, was cruel, oppressive, or abusive. Importantly, the government does not have to prove that the victim actually suffered physical or mental harm. The essence of the offense is the abuse of authority itself, so it is enough that the conduct, measured objectively, could reasonably have caused physical or mental harm or suffering.
This is why the phrase in the question, conduct that affects a subordinate’s well-being, points in the right direction but is not quite the legal test. The test is not simply whether the subordinate felt bad. It is whether the accused, exploiting the authority relationship, engaged in conduct that an objective observer would regard as cruel, oppressive, …