A CFI can only log 8hrs of in-flight dual given in a 24hr period (FAR 61.195). In my scenario, a CFI is required to be aboard a plane to satisfy insurance requirements. This CFI will put in a 17hr day, all of it in a plane that a non-student customer is flying as PIC. The customer has a valid medical, is current, and is appropriately rated in category and class.
The CFI might (or might not, he doesn't need the time) log a few hours of dual given to complete a checkout with this customer but then just sit there for the next ~15hrs staring out the window as the ground slides by and the customer racks up the hours he needs.
At the end of a very long day the CFI hands the customer a bill for 17hrs and is rewarded about as handsomely as a CFI can be rewarded. The CFI walks away with a fat stack of $20s and maybe 3hrs in the logbook.
Has the CFI broken a rule?