Too bad that this has happened. I don't think your friend should try to guilt him into changing his mind by throwing up the "look at everything we've done for you" bit or take back the car. As parents, you friend and her husband did those things because they love their son, not because they were looking for something in return. Sometime people think they have made a rational decision and often don't realize that there will hurt feeling.
I think your friend or her husband should ask their son if he can ask around and see if there is anyone who is not going to use both of their ticket. In any event, your friend or husband should let the son know that there are hurt feelings and the reason why. One of them should attend the ceremony and not sit at home and be angry.
edited to add
The conversation should be done over the phone if not in person. The son was a coward to send an email with this type of information.