Someone inserted inserted ain image into an imesage chat they were not part of

A client of mine was in a two-person iMessage chat when suddenly, an unknown person inserted an image and text into their conversation. Neither person in the conversation knew the number where the image came from. The unknown person who inserted the image and text was able to match the text of the other messages as if they were monitoring the conversation. How could either happen? Would airdrop make this possible? See the included screenshot.

Thank you.