Again, without more details, it's impossible to know for sure. He could have had a life - changing experience. Maybe he got into an argument with the person he was standing next to and it led to a new friendship or feud.
Since we don't know the context of this true story, we can only guess. He could have simply walked away and continued with his normal life. But perhaps there was something about that moment of standing next that stayed with him. Maybe he met his future business partner while standing next to them in a line. Or it could be that he was standing next to someone who was about to receive bad news, and his presence provided some sort of comfort or complication in that situation.
There are many possibilities. He might have witnessed something extraordinary and his life took a different turn because of it. For instance, if he was standing next to an inventor when they had a sudden breakthrough idea, he could have been drawn into that world of innovation. Or, if he was standing next to someone who was in trouble, he might have helped and become a local hero.