In the (g)olden days of the early 1970s, a passenger next to the exit window of a Gulf Aviation/Gulf Air DC3 would occasionally open it if the cabin got too hot and sweaty for his/her liking.
This happened in flight as often as on the ground. It was very easy; the window, which was really an emergency exit, hinged at the bottom edge and just fell out and down when the lever was operated.
The First Officer, or the Steward after the CAA insisted on cabin staff being carried, would only have to close the window and ask the passenger next to it, politely, to leave it shut.
It was no big deal. The window had a piece of coated wire attached to the top to pull it shut with. The airflow round it was at cruising speed, but fairly neutral in effect. I suppose the pilot could have slowed down and maybe side-slipped a bit to help, but I don't remember that being done.