This might be a stupid question, but I need to be sure.

A friend from a different is planning a vacation for a few weeks over in my country. He doesn't have a
laptop that can take the grand majority of games, so I offered him the option to use mine.

A bit later however I thought that if he installs a game I don't own on my account on my PC, the Galaxy client might freak out about it, or worse.

So I wanted to ask if there is any "best practice" for this? I was thinking of having him login with his account, but then MY games will cause the same issue for him (and I kind'a like to have my Steam and Galaxy running in the background in case someone shows up in chat).

Make a second Windows user account for him? Haven't played with that myself but wouldn't that make it pretty easy to set Galaxy up for him (and Steam, I guess) without messing up your own stuff?

I did consider that, but it would cause even more problems (since I cant check on Skype, Steam and other stuff I use).
So far removing Galaxy seems like the best option, but I wanted to know if there's anything more convenient.

Would it not be fine if he used the GOG Downloader instead?

Doesn't the Galaxy client detect all installed games (not just the ones through it)?

As far as my experience goes, you could install the gog downloader games in a different folder. There for Galaxy shouldn't detect it.

Galaxy won't freak out. It will detect the game, it will detect that the current user doesn't own it, and it will have a "Get it on GOG" button. See this for an example.

I don't know how it goes with Galaxy, but with Steam the Steam client seems to be shared between Windows 7 user accounts.

I have a PC with two different user accounts, and each user account has the Steam client (each user having its own Steam account).

The other user has only one purchased game on the account, but I see it as an installed game in my account too, when accessing it from my Windows and Steam account. I haven't tried to run it though so I don't know if it is merely visible to me, or whether I can run it too. And whether that is by purpose (maybe it is, it kinda makes sense as we are on the same PC after all...).

If both users have the game in their account, it doesn't matter who installed it, both can play it. If only one user has the game in their account, the other user will get a "Steam Family Sharing" prompt, to ask the first user for access to their library.
No idea how it goes if there are 3 users, with 2 having the game and one not having.

jam in a different hd problem solved for the duration of his stay ;p

Ok that makes sense then. As said, i didn't try to run the game (Geometry Dash) yet from my Steam account as I recall it wants an in-game (publisher) login as well (which I don't have, the other user has), plus I'm not really interested in that game anyway... Too simple reaction game to my taste, I could just as well play Flappy Bird.

That's actually good I guess.
My main concern is that I don't want to get any flags on my account or anything like that >_<