Its called a 'queef', or for the blokes here, a fanny fart. Go on, laugh. Sadly Ive never had the pleasure of experiencing one, so I cant tell you if they smell or not. All I know is that they usually happen when you shag quite hard and fast, and its just a small pocket of air that comes out.
Either that, or his bird gets so relaxed that she really does fart after sex. If thats the case, I wouldnt recommend anal sex - at all.