This is a very interesting thread for me. I have been faced with this exact situation this past weekend, as well as a few months ago.
Here's what I have done (let me say that I work in the capacity of QC Manager) - The first instance a few months back, the Level II turned in the film with a great amount of rejects. This is unusual for my 3 welders (the maintain better than a 98% pass rate). I started looking at all the rejected indications, compared them to B31.3 criteria, and found that most indications were nowhere near the rejectable size limits. I did not make the call to accept them. I simply questioned the Level II about his reasoning, and brought to his attention the acceptance standards. Come to find out, he had been on a week long job grading to severe cyclic (more strict guidelines), and was in that state of mind. He accepted his screw up and went back and regraded and accepted the film.
The next situation - We shot 128 welds and when film and reader sheets were turned in, there were zero rejects. This struck me as odd. Thats a whole lot of welds, and to have zero rejects just didn't seem right to me. I got to looking at all the film, and there were a tremendous amount of reshoots due to poor quality film and bad handling. I also had "questions" to the acceptability of 7 welds. I called in another Level II to re-evaluate. Out of the 7, one ended up being rejectable. I did not agree with his evaluation of some of the indications (see my other post on RT indications; IP vs NF), however, I did not feel I was qualified enough to "argue" with him. I simply stated my reasoning for what I thought it was, and he did the same.
I kinda stuck on challenging the Level II's evaluation sometimes. I work as the QC Manager for the manufacturer, and I feel it is the manufacturers resposibilty for the quality level of the product. Level II's are human, so mistakes and bad judgements can be made. But I feel if I dont agree with the Level II, all that I should do is "question" his evaluation. At least untill I reciever further training in RT film interpretation. I would feel more comfortable calling a reject that a Level II has accepted, than to say a weld is accepted when the Level II has rejected it. But as I said, for now, all I will do is question.