Now he said he wanted to buy Arma CO for DayZ so let's take a look on the bad side of this particular mod: It's also glitchy, it has a habit to lag, a lot, it's filled with all sort of bugs. It's also extremely hardcore, which can be a huge turn-off for people. In the most recent patch you start weaponless, foodless and water/sodaless, which basically means that if you are new you are going to die, a lot or you'll consider starting a sprinting career because that's what you will do a lot, because the zombies detection system is still a little unpredictable. Another thing to consider: Watching a gameplay of DayZ can be really different from actually playing the game yourself. You may see someones Let's play like Sidestrafe or Jefmajor and say to yourself:"OMG I want to do that too!" but then you might end up disappointed. It's also "Walking+Scavanging Alone", the game, so consider if you have somebody to play this with or not, because unless you are into it it's not going to be interesting at all to keep playing
So the risks are pretty high, so high that you might kick yourself if you waste 30$ or how much it is on your steam on the game. Thank god it's at least not a full-price title. My advice for him is do what I'm actually considering doing more so than anything, wait for the game to go on a deal or something. But hey it's your 30$ and you know your PC and net connection better then I, so take those things into consideration.