JIm, that is an odd one relating to the lady visitor to Oz.
We a have choice here, though it varies state to state, whether we pay for the Ambulance or not. There is available ambulance insurance for singles or families and is rebatable against your health insurance if you have any. Once over 65 you no longer have to pay.
Have never heard of any ambo asking the question, their job is to pick you up and take you to where ever you have to go. Even if you have no cover they will still take you and you will receive an account in time. All customers of the service will receive such an account. The only time I was ever in one I no longer had to pay but still got an account . On it the question is asked, how will you pay? If a senior, no payment, if you have ambulance cover no payment.
The only way the son may have been under insured would be if he was insured as a single and tried to use family rates.