You're right you never mentioned the gender in your opening post.btw who said that it was a guy?
I don't think you should confront this person or tell your bosses. You don't know what happened between this person and the cabbie, and fortunately no one was physically injured. I know it seems like you're being told to bury your head in the sand over this incident but can else can you do? What will you achieve? What disciplinary action would you want to be taken against this person? You never know if you said something everything could backfire against you. This is tough, I think all you can do is to try and maintain a strictly professional relationship with them. If this person senses a change in your relationship and asks you what is wrong then tell them what you saw.
Now if someone had called the police over this incident or someone was injured then I would give a witness statement, but it never came to that.