I found this thread to be an interesting read and thought I would throw in my 2-cents worth.
Gifts are rarely without cost. If I pay for the breakfast of the guy behind me at the drive through, the current mythology is that he will 'pay it forward' with an act of kindness to some other stranger. If I open the door for you, there is the expectation that you will say 'thank you'. If I let you merge into my lane of traffic after you have gotten yourself in a pickle, I expect you to wave or nod or smile and hopefully you will respond to other drivers with kindness and consideration. When it comes to gifts of significant financial value, the expectations can also be significant. Whether the acquaintance was repaying a social debt or creating one I cannot tell and it doesn't really matter. The gift created a debt or satisfied one.
To expect an MSP or client to be understanding and pay the shopper is unrealistic. Rarely will you get one cent more than you personally paid, and more frequently you will be underpaid because the tip you left exceeded the 15% or 18% 'guideline'. So the shopper in this case has 'paid' twice. Once by submitting a report on the meal and once with the social debt that has been created or settled. It is an unfortunate situation and one of those awful 'learning experiences'.
Under such a situation I would have emailed my scheduler and/or left a voice mail at the MSP that my shop had been compromised by the unexpected appearance of an acquaintance at the site. I would indicate that I was unable to appropriately complete the shop so needed to cancel. If they wanted proof I had been there, I would show the date time stamp on the bill as well as the location, but folded to NOT show purchases, table number, server, ticket number or any other information that might allow fictitious creation of a report. I would sacrifice the fee because $12 is not enough pay for an in depth report.