I need to test my application on a friend's mac. Not to disturb him, I thought I could connect me to his PC through SSH (with -X option), but to avoid opening the app in his session, a new session has to be started on his pc... Can this idea work? And if yes, how can I do it?
no. By doing
ssh -X, the windows will be opened in your local session and the user on remote host will not even notice.
By connecting using
ssh (even with forwarding X11) you don't open any graphical session on remote host, but the application run there are running on X-Server which is on your own computer.