I do not recommend buying RU key from him or anyone else selling them for that matter.
The reason is: You have to use VPN to activate a RU key to your Steam. Sure, you save a some $ BUT if you get caught (which you will be in a few days or in a few weeks), Steam will:
1) Permanently ban your account
2) Permanently ban you from using online services (checking friendlist, adding people) and playing online (multiplayer)
3) Remove the game from your account plus permanently ban you from using online services (checking friendlist, adding people) and playing online (multiplayer)
BUT. It's up to you if you want to risk your whole Steam account only to save some $.
I'd prefer not to.
Next time you should add the consequences in the first post.