Steam - Counter-Strike Source $5 USD (75% Off)
"Counter-Strike: Source blends Counter-Strike's award-winning teamplay action with the advanced technology of Source technology. Featuring state of the art graphics, all new sounds, and introducing physics, Counter-Strike: Source is a must-have for every action gamer."
When I bought it it said "To thank you for your purchase, we have given you free guest passes to share with your friends. These passes will let your selected friends play the full game for three full days. " So maybe convince your friend to buy it first to see if you like it!