Hi...
While I agree with part of the scenario big D related to, there may still be some question on legality.
Keep in mind that the nature and the character of the operation is of utmost importance and we, as Commercial pilots, are expected to adhere to a higher level of integrity and morality.
Granted, it is possible to engage in a flight where you may be operating in a grey area, but it may come back to bite you. Small professional world we operate in.
So this woman expresses a desire to go to the mud wrestling competition and you mention that you're already going and have made arrangements for a rental aircraft. You tell her that she can come along. Perfectly acceptable in the eyes of the Administrator. However, if this passenger pays for any part of that flight other than the pro-rata share, it becomes an illegal charter operation. She pays pro-rata.....perfectly fine.
I've always attempted to advise those interested that probably one of the most important criteria for determining whether a flight is legal or not may not be so obvious. You, as the PIC, may have to obtain a bit more information from all involved to make that decision.
Regards