These are some good suggestions. TF2 is free, and great casual shooter fun, loads of different game modes etc.
you just missed portal 1 + 2 on a steam sale, $6.74 for the pair, but from what I've heard fantastic fun.
Minecraft you can lose so many hours in. Good fun whether you play vanilla (which is no mods), tekkit, FTB etc.etc.
Natural Selection 2 is a must! Also at the very least switch out his motherboard with this one http://www.amazon.com/dp/B0055QYKQO/?tag=extension-kb-20
A free game mod I enjoyed playing many hours: No More Room In Hell.
A lot of cooperation and communication is key to win.
Yeah, I picked up the portal bundle for 5 bucks before on a steam sale.
Minecraft, with friends, is quite awesome. I was playing that game, and that game only for 2.5 years :P.
He's not using an FX-8320 or higher, so the motherboard I chose would work for him. If he were to upgrade, a motherboard like this would be much better:
http://pcpartpicker.com/part/asus-motherboard-m5a99xevor20
The VRMs have a nice heatsink, and they are Digi+, not analog.
Tech noob, that's a good price for that motherboard, I say go for that one he might want to stretch his legs with overclocking,
Don't worry about not being able to run games, When I made the switch to doing the majority of my gaming on PC I was using a £200 budget PC a £75 graphics card and using a £15 wireless logitech keyboard and mouse combo and I could still run Borderlands 2 at mostly medium settings.
I think that was the worst way to get back into PC gaming there is.
The only thing I can think of that hasn't been covered is that if you get an xbox controller make sure it's the wired USB version and not a wireless one or you'll need to get a receiver too.
As for games most of the stand out free to plays have been mentioned but Black Mesa is another good one to pick up http://www.blackmesasource.com/
A good website to check out is greenmangaming.com. They generally have a 20% discount code and on pre orders that goes up to 25%. The only catch is sometimes they might run out of Steam/Origin codes for a few hours with the more popular titles. I've never not got a code on launch day though so don't worry if their prices sometime seem too good to be true.
I don't think anyone has mentioned gog.com either. They're good old games and they sell as you might have guessed good old games. They take older games and make them run on modern PCs and they're pretty cheap too. They also do newer games generally without DRM too.
I'm looking to build a computer also and I don't know if i should use torrents. Are they safe? And do you know an easy way to do it?