Should i tell my friend we have a history test tomorrow or let her not study and suffer?

so on Friday i let one of my good friends borrow the book we're reading in English so that she could take it to class since she forgot hers (we get points off if we don't bring it to class) and she forgot to give it back to me that day. we had been assigned to read a portion of the book for homework, but... show more so on Friday i let one of my good friends borrow the book we're reading in English so that she could take it to class since she forgot hers (we get points off if we don't bring it to class) and she forgot to give it back to me that day. we had been assigned to read a portion of the book for homework, but since i didn't have the class until Tuesday i wasn't too worried over the weekend. anyway, i texted her this morning to make sure she had it to give to me and she wrote back telling me that she was really sorry, but she was struggling with her asthma and couldn't come into school that day. i was kind of panicking about the reading since we're usually quizzed on it, but i felt badly for her and knew there was nothing she could do, so i let it go.
then i checked my myspace and saw an update from a girl that lives in her neighborhood saying that she was skipping school that day and going to go tanning with my friend. my friend has sort of ditched me numerous times to hang out with this girl and she's kind of been the reason me and my friend have been growing apart the past few months they've been hanging out. i realized that she was almost certainly lying about being ill and that it clearly was not much of a matter to her that i was not going to be able to read my book, though i doubt she gave it much thought as she asked me to borrow someone else's book when she knows they would need theirs to read as well.
i'm really upset about this, and on top of that i'm a little bitter that she always seems to manage to do less work than me in school by copying or skipping or just getting lucky, and she is kind of smarter than me i have to admit, so usually pulls out better grades than me.
anyway, first period tomorrow we have a history test, and i know she doesn't know that today my teacher changed it from wednesday to tomorrow. part of me knows i should text her and tell her (she won't study she'll just stay home so she can take it another day) but the sort of petty part of me wants to see her face tomorrow when she realizes she has to take a test she didn't study for. what should i do? i'm also a little reluctant to tell her about it because it might give her the message that it's okay to basically screw me over and then expect me to help her, so what should i do about that if i do tell her?
by the way, i ended up being able to get the book at a kind of distant library, so i did get the reading done, but it was last minute and i'm still pissed at her lol.

so anyone have some advice? thanks in advance :)
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