It is usual, but not mandatory, for the Captain to sit on the left.
I've had several cases where the insurance aspect is considered a 'grey area' (from the insurance broker, having been asked for advice).
I would suggest this is because, in the event of an incident/claim, both qualified pilots suddenly decide they were not PIC and state they were only passengers. In this instance one could see why an insurance company would assume that the occupant of the left hand seat was the Captain.
Ensure that who will be PIC, and who (just) a passenger, is decided prior to flight (preferably record that somewhere in writing) and neither plan to change, nor change PIC in flight.
In the scenario posted both pilots' would be legal to fly the aircraft but one is unfamiliar with it and the other experienced.
The experienced pilot should be PIC and the 'unfamiliar' pilot a passenger (who will not log anything for the flight).
Because this is not an 'Instructional' flight the passenger cannot pay for it; however the pilot can cost share with the passenger - and, with the new rules, as long as the PIC pays at least one pence towards the cost with the Pax paying the balance then it falls within the rules for a private flight.