To answer the OP, I'm not sure what I would have done in exactly that situation. In a similar situation at the ticket counter, I saw a passenger just totally unload on an agent. I overheard the whole thing, the passenger was totally out of line, and I got the feeling that this could easily result in the passenger filing a complaint against the agent (that's right: I suspected that the out-of-line passenger might be the one filing the complaint). I offered my card to the agent and said that I had heard the whole thing, and that if needed, I'd be willing to give a statement as to what I saw. He politely declined, saying he was a supervisor and that it wouldn't be an issue.

I guess the way I feel in general about these things is that it's not my job to police my fellow passengers even if they are out of line (of course, there's always a boundary that can be crossed beyond which bystanders need to take action). But I do feel sorry for front-line employees that are harshed upon unfairly, and if there's something I can do, whether it be to offer to be a witness or just offer a friendly smile and a thank you, then I'm all for it.