Yes, Andrew did the right thing. Let me explain the issue though since the difference between passing messages and passing screenshots has been brought up.
Obviously, it is easy to forward a message to someone. Also, since when you forward a message you have the opportunity to edit it first, you can clearly doctor it. I would expect almost every user to realize they can do that. You can do the same with emails, after all. The natural assumption is when you have text in a window, it can be changed. On that basis, anyone with half a brain would realize that when someone forwards them a message they claim to have been sent from someone else, it may or may not be real. Same thing as emails and the internet in general; we all know that a forwarded email may not be authentic just as we know anything we read things on wikipedia etc may not be either. Therefore, the receiver will always have an element of doubt.
The anonymous poster points out that the same is true for screenshots. You can edit any image with the right tools and skills, and make it say whatever you want. But there is a key difference here, and that is in the eye of the receiver. While everyone knows you can doctor a message, I suspect most people only vaguely have an awareness that you can doctor an image. Some may even not know it is possible, but I think the vast majority know it can be done BUT would instinctively feel only a nerdy type or a hacker probably knows how. If you do not know how to do it yourself, it is easy to assume that the other players don;t know either. So if someone sends you a screenshot, my bet is it would not even enter the minds of most players on this site that it is not trustworthy.
I hope you can see that this would give someone who DOES know how to doctor screen images a great advantage if they were able to send screenshots. Think of the damage you could do in a FoW game for instance, where you could 'show' the other player exactly where other country units are...or where you WANT him to think they are. Look at the difference there from sending a message saying 'France has an army in Piedmont' or even forwarding a message 'from France' that says he is going to move his army in Piedmont into italy.
The key, therefore, is that in Diplomacy games, there must ALWAYS be an element of doubt. This is why metagaming is such insidious cheating. It strikes at a core facet of Diplomacy. If you can negotiate and operate in the 100% knowledge that you can trust what another country is saying, then it is no longer the game that it was designed to be. Even in the best alliances, there is always a little voice whispering to you that perhaps you are being snowed

. Take that away, and you break the game. Similarly, while you may know not to trust schreenshots, many others will tend to regard them as 100% true.