Is it possible to fake Gmail Inbox Screen and navigate as if it is real?

Is it possible to fake Gmail Inbox Screen and navigate as if it is real?

I have a weird problem.
A friend of mine showed me on his computer (Mac/Safari) a Gmail screen
with a fake email in it (I know for sure that it is a fake mail as it was
apparently sent from my ID, on a past date). He was able to navigate on
the screen just as you would in real Gmail. He clicked on the fake mail
and it opened as a regular Gmail window. The URL in the browser was seen
as that of real Gmail (nothing phishy in that at least). He didn't let me
touch the computer and all along he was navigating.

My question is:
1. Is it possible to fake something like this?
2. If yes, how did he do it? I don't need implementation details but a
high level response to help understand what he did to make it look so real
would be helpful. Is it easily implementable?