Buying someone a Steam game without a Steam account

Instead of creating a temporary account or giving a gift card, you can use an authorized reseller site like the humblebundle.com or greenmangaming.com to purchase a Steam key that your friend can redeem. Often these kind of sites have the added benefit of allowing you or your friend to select either a Steam or GOG key (or just download a DRM-free installer directly for some games).

Note that in my experience, some people have trouble understanding how to redeem these keys, but if your friend is familiar with Steam or GOG, they should be able to figure it out. These sites do a pretty good job of explaining how to redeem keys.

("Artifact" is a Valve game and therefore will probably only ever be available on Steam, but for others looking for a way to do this with other games, I think this is the better option.)

IMPORTANT: Make sure that you only buy keys from a legitimate authorized reseller! There are illegitimate sites, such as G2A, that sell illegally (or questionably at best) acquired keys. These keys have a high chance of getting revoked, so it is very important to check to make sure the site's legitimate.

There are two ways that come to mind. 1 is to give him a gift card (as Frank mentioned in his comment), the other is to make a new account and gift it to him through it, and then delete that account (which is probably more complicated since I believe you will have to re-friend him through that new account)