Your friend points out that your fly is low. Do you:
Turn around quickly and see to it, thus potentially displaying your shame to the rest of the office, if they all looked up at the same time and could even tell what was going on
You take care of business there and then, scandalising the only other person who actually knows what's occurring.
Also assume that while you are enjoying pleasant banter with your friend, you don't actually have a semi IE it is politeness rather than anything that will require referral to HR.
The non-trousers version of this would be different, as presumably "You have tucked your skirt into your knickers" would by default be happening behind you. If you managed to do so on the frontward side, you might well want the hidden third option "burst into flames immediately".
[Edit: I now regret this]