This case should be winnable, but obviously there needs to be a detailed account and any supporting evidence you can get (the tribunal can't just take your word for it, otherwise anyone could just make up that their passenger was having a fit).
You say "we", how many passengers were in the car?
How old is the passenger?
Are medical records available to confirm their medical condition?
Do you have the contact details of "Lady from front of car"?
Do you have receipts from the pharmacy / shop?
What did she buy?