Have you experimented with leaving the window closed during the day and the curtains pulled across? Sometimes that keeps a room cooler than opening a window, by keeping the heat of the sun from getting into the room. Depending on the circumstances, it might help.
This is worth repeating. If the window is getting direct sun all day, and your roommate leaves it open all day, I'd be willing to bet that your roommate is inadvertently heating the house up - it could be doing the exact opposite of what he wants!
When we have hot days I leave the windows open only during the cool evening/night/morning hours. By strategically placing window fans or box fans in various windows, I pull cool nighttime air in a few of the windows and vent the stale warmer air out of others. Then in the morning when it gets as hot outside as it is inside, or when the windows get direct sun, I close all of the windows and pull the shades closed. It keeps the cool air inside, and having the shades pulled slows how quickly it heats up - you don't have hot air coming in an open window, or direct sunlight through the window heating the house up. You can even get shades with a decent R-value to help insulate, keeping the hotter sunlit air away.
I'd guess that my house is a good 5-8 degrees cooler inside on really hot days than before, now that I've started cooling it this way (nights in my area are usually 20 or so degrees cooler than the afternoon high - obviously the fans won't do much if the daytime and nighttime temperature doesn't change much, though closing the shades during the day will still help). But you have to close the windows and draw the shades during the hot day, or you'll lose all of that benefit, and perhaps make it even hotter.
I have a ceiling fan, and I think it is useful for pushing hot air down during the winter, but I think it's pretty worthless for me in the summer (I do reverse the direction of rotation summer vs winter). It gives the illusion of being cool because it moves the air around, but it doesn't cool the air at all. Some people like that but it doesn't work for me. I'd rather have a fan in the window pulling cool evening air in.