I have to admit I am surprised at the other answers. In my view this is cheating plain and simple. The student is passing off someone's else's work as their own on an exam. That is cheating. How they got the answer key is irrelevant.
My first paragraph is a little strong. There may be some doubt that the student actually used the answer key. But I think a strong case can be made—at least for further investigation—since all of her answers matched the answer key. If the student is a strong student, and has a history of getting high marks, that increases the amount of doubt. If this test score differs substantially from her previous exams (or differs substantially from the performance of the highest scores from other students on this particular exam), that decreases the amount of doubt. Was she the only person to ace this exam?
Her account of the situation doesn't seem credible to me. First, there is her changing story (at first she said she didn't notice the bolded answers, then she she changed her story to say she thought they weren't correct). The idea that she thought "the bolded answers weren't the correct answers" defies logic. Why would they be bolded? Had any other exam in your course ever have some of the answers in bold? Isn't it odd that only one answer per question was in bold?
Why would this student say she "didn't think the bold answers were correct" in spite of the fact that she marked those answers on her scantron? Doesn't the fact that she marked those answers indicate that she did think they were correct? Isn't she contradicting herself?
Either way, this situation is suspicious enough that it should be reported as a possible infraction to the academic integrity committee. Let them decide if further investigation or other action is warranted. You don't need to be be both judge and jury. But you do have a responsibility to the school and to the other students to at least report activity that is very suspicious.
Based on some of the comments, I think I should add: This is not a zero-sum game where either the student OR the OP is at fault. They can both be at fault, but for different things. The OP is at fault for mistakenly releasing the answer key. But there is a very strong appearance the student attempted to capitalize on that mistake to get a grade higher than she would have earned on her own. If the student didn't recognize that the correct answers were in bold, it's unlikely her scantron would match the answer key, even down to the mistake on the answer key itself. If the student did recognize she had an answer key, then she is at fault for using it. Merely getting or having the answer key is not her fault. Using it to try to inflate her grade is the problem. If she knew she had the answer key, then she would have known that was a mistake, and she should have brought it to the attention of the proctor (as the other student did). Getting the answers by mistake does not lessen her obligation to take the test honestly, so that it reflects her mastery of the subject. (The previous sentence is what seems so self-evident to me, that I started the post stating that I was surprised at most of the other responses.)
Her only possible excuse is that she thought the test was supposed to be that way. Or that she recognized the bold options seemed to be the correct answers, but she tried to ignore those and answer the test on her own. The first reason is not credible. It fails the laugh test. This is not her first rodeo. She knows how tests work.
The second reason is more believable and IMO a valid excuse. Except in this situation, it's not what she said. And it strains credibility since her scantron matched the answer key, even down to the mistake on the answer key.
Nonetheless, she deserves the chance to explain herself. The Academic Integrity Committee (which at most schools has student representation) is the appropriate place to do that.