

What magic pressure differential are you referring to? A/C recirculates indoor air and has slightly positive rooms and slightly negative rooms, but will always be net ~1ATM of pressure. If his room has slightly positive pressure and he opens the window, then guess what, hot humid air is being pulling into the house in other people’s rooms, hallways, bathrooms. This would also explain why he doesn’t experience the issues it would cause.

I don’t have to know a single one of those details, because I know for a fact that AC does not create a magic pressure differential within a house relative to the outside that could possibly negate hot humid air intrusion. That is your assumption, not mine. When the window is open the temperature differential across the opening with cause convention (mixing of the air). If his room actually is positively pressured enough relative to other rooms, then he is causing hot humid air to be pulling interior at other locations around the house.
I have sympathy for OP, but in 90F, that sucks for whoever is paying for the AC.