Don't care that his insurance has this as a writer, http://web.law.und.edu/LawReview/issues/web_assets/pdf/87/87-2/87ndlr233.pdf
1:) Does your state recognize this as a defense, if not it can not be raised.
2:) The driver must prove that such a condition existed, if they have had previous seizures and claim a seizure the defense won't work because it was foreseeable, insurance back on the hook. Ask your agent to provide you with the documents that prove the condition, the medical records from the ambulance . . . see if he blinks, can't tell you you can't have it they must prove it to claim it so you are entitled to it.
3:) If their car insurance doesn't cover it, do they have homeowners insurance?? They might be next on the hook.
4:) They are still liable to make you whole, no other way around it. I would not get a lawyer until you absolutely need it. You don't want them to get the 1/3 cost of repair and be on the hook for that if you don't need to.