You are asking two related but distinct questions here. First:
Is it cheating for Shion to see the exam taken on Tuesday April 12
2016?
No, unless she promised that she would not see the exam (for example by signing a form to that effect as mentioned in The Fire Guy's answer). And it is not clear that there is any reasonable basis for the professor to ask her to make such a promise, unless the professor is giving her the make-up exam as a special favor that he/she is not required to do.
And second:
Student taking a make-up exam sees exam she would've taken: Unethical?
Yes, this could very well be unethical, but it is the professor who may be behaving unethically, not the student. I see several issues here: first, the professor allowed all the students to keep their copy of the exam, which makes it easier for them to share that information with Shion. Second, the professor is planning to give Shion a make-up exam in which knowledge of which questions were on the original exam could give her an unfair advantage (at least I'm assuming that's the case, otherwise the question wouldn't really make sense). And third, the professor is planning to give Shion a make-up exam which is "significantly more difficult" (note that although this goes in the opposite direction from the first two items, it is still unfair and the three sources of unfairness do not necessarily cancel each other out).
All of those things point to the professor not really thinking through very carefully his or her approach to fairly assessing Shion's performance. At the very least it seems like sloppiness on the professor's part, and depending on the level of negligence involved could potentially rise to the level of unethical behavior.
Summary: It is the professor's responsibility to design a make-up exam that assesses Shion's performance and knowledge as fairly as possible compared to the other students. Different professors may take different approaches to doing so. Some would find a way to ensure that Shion cannot look at the original exam; others would write a make-up exam that is of the same level of difficulty as, but different enough from, the original exam. Creating a situation in which Shion can easily have access to the original exam and benefit from this information is a recipe for trouble and probably means the professor is not doing his/her job as well as he/she should be. In any case, Shion could only reasonably be accused of cheating if she made an explicit promise that she would not look at the original exam and then did so anyway.
