Games have no serial or way to know if your account has the game.

They are stand alone. You COULD give a copy of the installer to every person you meet. But you would be breaking the law and be a very bad person.

You can give your friend the installer if he has the game in his account to save him downloading it.

there is no serial code, no drm, no account log in.
but to respect the eula you cannot play it at the same time of your friend (just think like you have a cd, you cannot play it if you give it to your friend). anyway there is not a drm to stop you (you don't have to do that anyway).

It's GOG, you don't have to activate any games to be able to install and play them. That being said, for multiplayer purposes some of them require cd-keys. But you haven't mentioned what game you're going to play so it's kinda hard to tell. ;)

The installers are always the same, no matter what account you download them from, and if a serial key is required, you'll find it in your library on the site.
So, yes, if both of you own the game, there's no problem in just downloading the installer files once and then copying them to the other PC via USB stick, external harddrive or even a burnt CD/DVD.
You have to get the manual installers, though. This doesn't work with installations through GOG Galaxy.

Wahou, thanks everyone

That's some nice and clear answers.

we will do that then

have a great day

Mat

Took me a couple of minutes to find, but here's gogmix of games requiring serial numbers with helpful info, compiled by Grargar in this thread.

You didn't read the OP properly. They both bought the game. All he's asking is if they have to download the installer twice (from their separate accounts), or they can just download it from one of their accounts and copy it from one machine to the other. I'm assuming the OP brought his laptop with him to his friend's house, or the other way around.

Sorry, I didn't undestand.