Not your everyday real-life story. Client: "Integrate this (see attachment)". You, "No, they have first to hand over the source code in order to allow me auditing their software." Client: (gone).

As for the real-life example given above: The third party is billing the client only a few bucks, since the real business is in profile building and exchanging profiles. So, are they expected to hand out the source code for 5 bucks or so? Probably not. Who is to lose? Everyone visiting the website.