RE: Battery and starter changed - Intrepid won't start!
Classic dirty ground wire. When you jumped it, your friend probably grounded on the frame, not the battery. That's why it started. Remove the ground wire from the battery, follow it to its end, remove, clean and reinstall. Let me know how it goes.
PS it could be the cable is bad, even the hot cable...if you shifted or moved while strting. Make sure the battery terminals are clean, but from what you said, I'm leaning toward the ground.
PPS, I'd start seeing a new mechanic. He could/should have tested the starter. You can have it tested for free at auto zone while in the car. For a full test, it needs to be removed though (still free).