well said bro CHiNO730
TS, sorry to said, ur friend really noob i think about pc cooling system. In addition to what bro CHiNO730 said, another well known problem for a case-less pc is dust. Believe me, u dun want to see ur pc covered in dust just for few days usage, and i dun think u have all time available to clean it. And do u prefer to see ur pc (which u spent hundreds to thousand bucks) dun have any protection for it? IMO, better to have a casing, a cheap 1 also can, as long as u know how to set up a good airway flow into that casing. How to do it? There are lots of topics that discussing about that here, just spent some of ur time to read them and applying to ur pc setup.