Well, if a diaper boy poops in public, it's important to act fast. Check if there are any diaper - changing facilities nearby, like in a shopping mall or a public building. If not, you may need to use your car if it's close by. In case of no such options, you can ask for help from the staff at the place you are in. They might be able to direct you to a suitable place or even provide some basic supplies if they have them. Also, don't forget to clean the baby's bottom properly to prevent any skin problems.
The first thing is to stay calm. Then, find a private place like a restroom as soon as possible. If it's a diaper - changing station in a public restroom, that's ideal. Have all the necessary supplies such as diapers, wipes, and diaper rash cream ready. Change the diaper quickly and clean the baby thoroughly.