It seems like they are living in a closet-sized room. Do you know for a fact there is even an intake vent and that OP doesn't block it? You have no idea what the situation is. I live in a significantly more humid climate and A/Cs are never installed or maintained properly, even in some schools. Hot and humid air will only be true at some points of the day for OP, who lives in California.
The only reason we are even talking about humidity damage is because the person who responded to OP immediately took the landlord's side and loosely accused OP of feeling that everyone is against them right off the bat.
They want their window open, we should tell them their rights, help them understand the risks and potential mitigation strategies, help them formulate a defense... most people have zero issues doing what they want as long as they aren't causing damage. Opening a window is reasonable because the A/C is too cold for them.
If you look at OP's previous thread, their landlord seems fairly unreasonable, even considering we are getting a one-sided account of events.