How can I delete a message I've sent to a friend? Can I delete at the same time in mine and in my friends inbox?
In Gmail, this is not possible unless you have already enabled the Undo Send feature. The official support information about how to do this is here.
Even so, you have a maximum of 30 seconds (10 second by default, but you can choose up to 30) to recall the message. After that, it is sent and you cannot recall it.