Unless you have a large fan blowing directly on your setup, you will need a pc chassis (assuming you mean CASE?).

A well designed case will have properly placed intake and exhuast fans. Basically they create pressure/vacuum in the case to get air moving through your components. This airflow is necessary to cool your components and assure longevity.

Your friend, sorry to say, does not really know what he is talking about.

Without the case, the components will dissipate heat at a slower rate, since there is no airflow around the heatsinks to evacuate the air.

If you don't know what I mean, try taping a few pieces of paper inside a well ventilated case or light a cigarette in front of the intake fan. You will see the direction of the airflow. Remove one of the side panels, and do the same thing. Less airflow. Now take the whole thing out of the case and try again. No airflow.