Am I a backstabber or just a concerned co-worker?

Here is my dilema. I am in an intern program that had over 50 applicates for 5 positions. The first day I was happy to meet the other four interns. Two of them graduated right before me, one grad was from another class but I worked with him so I knew how smart he was, the other nurse is from another town and has proven how very smart she is. The four of us were doing great in our classes and orientation but this one nurse just seemed to be lost. I tried to help but she just was kind of mean to me so I started to just ignore her. The others in class started to make fun of her. The instructor told us that we needed to cut her some slack because she has ADD. My son has ADD so I can understand how hard it is to sit there for 8 hours and try to pay attention especially with a handicap such as ADD. She told us she was on medication for her disability so I cut her some slack when she constantly fell asleep in class, text on her cell during lectures, and just got up and disappeared for over an hour as she said she had to go to the bathroom. I told the others to grow up and quit making fun of her when she asked questions like 'What is PEEP?, Where does the chest tubes go?, Why do we as nurses have to give baths...isn't that the techs job?'
Now here is my dilema, today she told me that she lied, She doesn't really have ADD. She just used it as a sympathy card to get the job and look like she was a super nurse that has overcome all the hardship of having a disability. If I talk to my manager and tell her this my manager is going to think I am being petty, I am being mean, or I am trying to get her fired so that I can apply for the positions that the other nurse has applied to and get them without the competition. The Human resource manager, and the other managers are not going want to admit that they were fooled by this slacker and I fear that the backlash will be only on me.
What should I do?