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To quote directly from GOG's FAQ:
Can I install one game both on my laptop and desktop PC at home?

Yes. We do not limit the number of installations or reinstallations, as long as you install your purchased games on PCs in your household. So yeah, if you've got a render-farm in the basement, you might actually break the world record for the number of legal Fallout installations in one household. However, if you think about installing your game on a friend's PC or sharing it with others then please don't do it, okay?
Applying to this situation, if you own a second PC that you let your dad use (or just want to let him play it on your PC) then go for it. If your dad lives across town and wants to use your installer to put the game on his own PC then he really should buy his own copy.

It should also be noted that there aren't any technical restrictions on the game to prevent you from installing it on as many PCs as you want, so this is really just a matter of doing right by GOG since they tend to do right by all of us customers.