I've done beta work in fandoms I know nothing about! It's sort of the same thing. As a beta, you have to learn to be objective. It's not about "Do I like this story?" so much as it should be "Is this a well-crafted sentence and does it fit with the sentence before this one?" and that is where the fine line comes from. It is easier to give honest feedback when you know the person better but it is possible to give feedback even if you've never met the person or read anything from them before. Good feedback should always be motivated by the writing and not on likes or dislikes.