My sons car was stopped in a parking lot when his friend opened the passengers side door. As another car was squeezing by, and hit the door. Who is at fault?

Boith parties - the person who opened it wasn't 100% sure that it was safe; the person pulling in had a responsibility to avoid any obstacles and have his/her car under control.

Normally they will find both parties at fault. Depending on where the point of impact was, if for example the car was already well pass the door and they opened the door into the back side, then perhaps the person who opened the door would be more at fault.

But he needs to merely turn it over to the insurance company and let them work it out