While I haven’t ever been in this situation myself (I tend to stick to critiquing genres I enjoy myself, because it is easier), but I can see two ways to handle this.
#1, like you mentioned, is to let her know that you’re not the best person to critique it because it’s not a genre or a style you’re familiar with/you enjoy.
#2 is to focus on the things that are the same across genres. Things like, were the characters likeable? Were they relatable? Did the writing paint a clear picture of the setting?
Even if you go with #2 (which I would recommend, personally), I’d also recommend telling her that you’re not the best choice for critiquing her writing in general because it’s not your genre/style. I think it could be beneficial for you to practice critiquing a genre that’s different from yours, but letting her know after this critique that you’re not the best fit will help you avoid situations like this in the future and will tell her that she can find more helpful feedback elsewhere, so it’ll help you both.
Fantasy/dystopian/sci-fi author. Mythology nerd. ENFP. Singer.