I was really surprised to get a beta request for The Mentalist. I don't follow the show. The writer told me that my lack of interest in the show was one of the reasons she wanted me to beta the story for her.
She said that besides respecting my opinions she thought that I - as someone that read fanfic but did not follow the show - would understand what she was trying to accomplish but not be influenced by preconceptions or fill in detail that wasn't really present.
I told her that although we had discussed stories and exchanged recommendations, expressing an opinion about what I had read was not the same thing as evaluating the story; that serving as a beta required looking deeper into the story. She kept pushing. So I finally offered to provide an example, and asked her to pick a story from several we had recently discussed (by other writers) for me to beta so she could see the difference between me as fan and me as critic.
She did. And was horrified at the results. She accused me of a lot of things but seemed to be the most upset by the hypocrisy. She could not believe I left upbeat and positive feedback to the writer when - based on my beta report - I thought the story was awful. I reminded her that I had attempted to explain the difference between reading a story for enjoyment and being a beta; my attempts at logic did not go over well.
On the other hand she did decide that I would not be the right person to beta her work. And she'll probably start talking to me again in a week or two.