The concept described here is what us homebuilders call “make up air.” In a tight envelope, when you need to exhaust air, you have to forcibly introduce fresh air to make up the difference. Otherwise your envelope (home) will sit in a vacuum and air won’t move anywhere.

Usually, there is only one window in a bathroom. If you have a fan, turn it on and open the window and it should disappate quickly. If you only have the window and no fan, punch a hole through the drywall to the next room, preferably close to the baseboard for optimal airflow. I assume you are at your “special friend’s house” and trying to hook up. Wait a few minutes and it should clear. Hope this helps!