No, no. She's not "at fault." Well, you could argue about PERSONAL right n wrong but... ANYway. Look at it objectively. It IS illegal. I know many people who are very uptight or black and white about right and wrong. About legal and illegal too. It's a little more philosophical people who see the gray areas. She got scared, some people have to tell others when they're scared.
And no no no! Do NOT deny. It a. makes you look like an ass b. erodes her trust in you. Coming "clean," even making speeches about how gee, you want to get stronger and about the firefighter position, you're worried about being a great candidate vs. a good candidate, it's a good job where you could support her and she could be proud of you, blah blah... PLUS - admitting you did it and that you "fucked up" because you wanted to try it and so many other people are trying it and you're curious if it would help you ... makes you human. A very endearing quality. You could really snag her with the whole human/job so you can be proud of me angles.
In time, she'll learn. Everyone lies. You love her. There's going to be shit about her that she hides, maybe not considering them "lies," there's going to be things about each of you that you won't want your sig. other to know, and everyone wants some privacy about some stuff. Sorry, no one's perfect and some people have imperfections that'll hurt/annoy/disgust people they love. But it's part of you. So you hide it.