While this sounds like a reasonable use of the term, it is not, and no lawyer would say such a thing. Contributory negligence is actually an affirmative defense, meaning that it can be raised to help a defendant when someone else sues them; it is not a reason to start a suit against somebody. It is a common law doctrine (not written down as a statute) that says if the plaintiff's negligence contributed to his own injury, then he cannot recover from a defendant, even where the defendant's negligence was the principal cause of the injury.
Imagine that Josh was test-driving a giant Hummer SUV, driving 20 miles over the speed limit, curving back and forth across the road. Donna was driving her Prius toward Josh, driving below the speed limit in a perfectly straight line. She and Josh collided when he crossed the center line, but at that time she was looking away from the road toward her pocketbook to pull out her ringing cell phone. Of course Donna would want to sue Josh, but she would be barred from recovery under a theory of contributory negligence because her negligence in looking away from the road to get her phone contributed to the accident, even though most of the fault lies with Josh. That result angers many people, and in fact, because the doctrine is so harsh, many states no longer recognize the defense of contributory negligence or have created so many exceptions that it is no longer quite so harsh.
Contributory negligence might come up in the President's case, but it would be his defense against the claim Sam is concerned about. The President might argue that even if he bore fault for the accident, he has no legal liability because the contributory negligence of the driver who chose not to wear a seatbelt defeats any liability on the part of the President. For more information, see Law.com's dictionary entry for contributory negligence.