Considering that there was a party being organised by David's family - it is somewhat understandable that he went to the market. He was being thoughtful of his mother and wanted to assist her. However, it might have indeed looked strange that he was simply hanging around. Hopefully you could apologise to calm down the situation and that would suffice.

Planning to prank David again migh be a good lesson for him and hopefully he is not so gullible otherwise he will indeed learn a lesson from you.