That's not really a big deal seeing as how I can just click on the big ol' GMAIL button in the top-left and go straight to my friend's inbox if I really wanted to see his email anyways.
What is different is that you are intentionally spying on your friend if you did that. Here, your friend is unintentionally revealing contents of his inbox to you.
considering the gmail results are hidden and require clicking on a link to show them, its not much different than opening up your friends email now is it?
