By convincing him to buy the product key from GOG.com

Picture added for better visual representation of myself.

The Product Key? I love that game!

Yes, very much so. Can anyone say, "GOTY 2015"?

On a serious note, he actually bought Chroma Squad. Looks like a fun game.

Here's hoping he doesn't get burned by ultra-slow updates for it here, since it's a brand new game. I know some people can't stand it when a game they bought here takes much longer to receive patches or other updates than the same title on Steam or other client-based services. (Hopefully it's a solid game to begin with, though, so it won't need so many updates.)
Now you only have to get him to buy an actual Good Old Game from the catalogue here. :P

I officially endorse this action :)

Congrats! Did you use lube?

Given the 'na zdrowie' below your name I'm guessing you got him drunk first.

There's people out there who don't buy indie bundles? D:

I've probably got a ton of DRM-free installers outside of GOG, I just never bothered to keep track.

Now, if you can convince him to play Teenagent...

Haha, yes. Usually, but I was able to persuade him with dat DRM-Free knowledge! Thank you.

That can be arranged. Do you like Polish kielbasa?

Waaah! I wanted to pop my friends DRM-Free Cherry too, but all of my (remaining) friends are Steam's beaches... Well done! Keep up the good work!

I hope you used protection!

Very good choice!
I hope he sticks around. :)