Well here becomes a little technical but the bottom line is ... the person riding the motorbike did not have an an motorbike driver-license. There are a few lessons from this story.
Fist of all you should ALWAYS have a travel-insurance. And check if your travel-insurance (or your healthcare-insurance) is valid for holidays in Thailand. And check if it is valid for the things you want to do in Thailand .... motorbike riding, diving, other dangerous sports or example bungy jumping. Do not assume that your travel-insurance will just pay for everything.
And secondly remember that ALL scooters in Thailand are motorbikes. And for motorbikes you need an motorbike driver-license. Yes, the rental company does not care much as YOU as renter are responsible for ALL damage to his motorbike anyway. Most (if not all) scooters / motorbikes have only very limited 3th party insurance.
And it can no be said enough ..... while riding a scooter / motorbike at least wear a good helmet, good cloths (better wear also a good jacket) and good shoes. Better also wear gloves. The roads of Thailand are not Disneyland! As a matter a fact the roads of Thailand are dangerous. And another things .... drinking & driving is just stupid! Yes, also on your tropical holiday in Thailand.
And remember .... Asfalt is the best tattoo remover!