It happens. We forget to zip up or a button pops open. I am frequently asked how to handle these embarrassing moments.
As long as you’re not perfect strangers or just been introduced, quietly and discreetly tell the person their fly is unzipped. This is usually comfortable between two men, but can be a little embarrassing when the conversation is between a man and a lady. I recommend you endure the moment of embarrassment and spare an individual extreme mortification of discovering it on their own after a period of time.
If you have been recently introduced, ask a mutual friend or acquaintance to inform the individual.
The same advise goes for spinach or lipstick on the teeth, food on the face, or make-up smudges.