A landlord's wealth derives from being a middleman between the tenants and the property, not from the landlord's own labor. That wealth is quite literally the whole point of being a landlord in the first place; if the income from rent didn't significantly exceed the value of the labor your FIL performs as "maintenance", then why would he bother with dealing with all the headaches of property ownership if he could instead just be an employee of the tenants themselves (said tenants forming e.g. a housing cooperative)?
That is: there's nothing a landlord does that the tenants couldn't do themselves (whether directly or by hiring someone to do it on their behalf). It is therefore not labor alone that defines the relationship between a landlord and one's tenants. Your FIL may be "the hardest working person [you] know", but he's still a middleman in the ownership class.