Dave's friend Paul should have known that he needed a Thai Driving Licence or an International Driving Permit together with his foreign Driving Licence. That being said the regular fine for driving without a valid licence is only 400 baht (see
picture I found on internet), up from 200 baht.
In December 2017 a new form of traffic ticket is being introduced (see pictures from Bangkok Post).
Note that one of the possible offences is: Beyond the stop line when waiting for the traffic light. Many motorcyclists, also farangs, seem to ignore that. I find this strange as this rule also applies in Farangland.