before the psu died ,i tried to power it on but every time the pc shut itself off after a couple of seconds..
As it should. Your friend needs to replace it with a quality unit. A can of compressed air would not have enough pressure to damage a quality unit (from the air itself).
However, here is one way that I can of compress air damaging a good power supply:
- You take the power supply out of it's case and place it on it's side. You keep a constant hold on the can of compressed air, to the point that liquid forms on the outside of the can. Liquid drops into the opening on the side of the unit. The power supply is sent 120v from the wall before the liquid evaporates.
If I were in the situation with a good friend, I would at least offer to split the cost of a replacement unit. Whether or not you were the cause, you were the last one to touch it before the burn.