GR00T: The problem here is it's not obvious how you can run games without Galaxy. If he installed the game and got Galaxy unintentionally, then Galaxy will launch when he tries to start the game. If he's not logged into Galaxy, it will tell him he has to do so. The impression anyone that's not familiar with it would be just like Steam: they'd assume you have to log in to play the game.
So sure, Galaxy's optional and it's easy to run your games without it. But GOG's done their best to not make that obvious. Someone that doesn't hang out on these forums much will likely be completely oblivious to the fact you can run your games without the client.
Actually you install a game from your account on a friend's PC. Their Galaxy will recognise it (and tell them, they can't "connect" the game. But you can still play the game (from their Galaxy!) and even see game time and achievements.
That's what I did showing a friend Dying Light when I was at his place (using classic installer from me, but launching with his Galaxy). Convinced him to buy the game :-) Win-Win - DRM-free FTW, even with Galaxy.