I would talk to him and tell him that is not acceptable behaviour and not professional . Or something to that effect. Or if he /she is always doing stupid stuff. I would tell the boss. If afraid to tell the boss write them a note anonymously. The boss should be alerted so they can watch this person. I wouldn't work with this person . They have gone way beyond the norm. We have all had mean customers but really.
The first thing to do would be to ask your colleague to make another sandwich. If they are reluctant to do so, bin the sandwich with the fly in it straight away, tearing it so it can't be served. Then discuss the matter. If necessary, make another sandwich yourself.
If a customer complains of becoming ill after eating the food at your workplace, all staff will come under scrutiny; it'll be too late by then to put your hand up and report your co-worker, so tell them right now that unless you're certain it won't happen again, the matter will be reported.
You could be held liable for damage if a customer is affected by the food and it turns out you knew the food was being tampered with and took no action. But this shouldn't be your main concern: you should be above all concerned for the safety and wellbeing of your customers.