This situation might be because the ambulances are operated by NHS which is also UK health insurance, therefore for citizens no charges apply. Similar thing in Germany: the ambulances directly bill w/ health insurances, nothing needed except the insurance cards.

Now, the NHS may not charge German patients themselves for an ambulance ride in UK but either quietly deal directly with the German insurance to get the cost back, or eat it entirely because the administrative overhead would cost more than the fees paid by German insurance would actually bring in - while the German ambulance operator might have decided to put the administrative burden on your friend.

tl;dr: international health care insurance/refunds are weird, avoid if possible...

In the US, an ambulance trip can cost thousands. Also, if after being treated / examined, your insurance decides you didn't need the ambulance, they often won't cover it.