Sadly, flat tires are a fact of life in driving cars, and you as a guest are responsible for a flat tire that occurs during the trip, as well as any related damage that may result from that flat or damaged tire.
However, a guest will not be held financially responsible for replacing the tire if it is professionally documented that the flat is the result of a tire that was either 1) defective, 2) had been subject to excessive wear such that it was unsafe to be driven as of the start of the trip, 3) was not properly inflated when the vehicle was provided to you, 4) had tread depth less than 4/32th of an inch, 5) had signs of dry rot, or 6) was more than six years old.
In all cases, the individual protection plan selected by each party would be in effect for the tire claim. If the guest has selected Premium, Basic, Minimum, or Decline, the waiver from the protection plan will apply. If the host has selected Premium, Standard, or Basic, the relevant deductible will apply.
In the event of a flat, the guest should contact roadside service to change the tire and replace it with a spare, and then coordinate next steps with the host. If the tire can be repaired, it might be best to do so. If the sidewall is damaged or a repair is not possible, the guest is responsible for replacing that tire (without any reduction or pro-ration based on mileage) but is not responsible for replacing any of the non-damaged tires. The guest must get the host’s approval for a replacement tire and must replace the tire with one of equal or greater quality.
If both the guest and host are unable to work it out as stated above, the individual protection plan selected by each party would be in effect as part of their damage claim.