You may want to pick up a book on OCD to give you a better understanding of the disease. The classic one is The Boy Who Couldn't Stop Washing by Judith Rappaport, although there are others.
One analogy i can give that might help you understand, have you ever had a song stuck in your head and you just can't get it out? OCD is like that, but with thoughts. It just gets stuck in your head and won't let go. Its like our brains are sticky. And let me tell you we don't WANT to be that way

The trick with OCD, and something thats hard for people to understand is we can't just make the thoughts stop, thats the whole problem. Even if we KNOW they are irrational or stupid or just don't make sense, logic isn't enough.
Dwelling on the thoughts, trying to argue with them, to logic yourself out of them is actually one of the worst things you can do as an OCD'er since in reality what you are doing is continuing to focus on the topic, keeping it in your mind. Hopefully she is going through therapy and/or on medication, but if not you might be able to help her by picking up a good CBT (cognative/behavioral therapy) work book and going through it with her if she's willing. It might be a good way to help build a relationship by working through a struggle together.
Its really commendable that you are taking these steps to understand what she is going through, believe me as an OCD sufferer that means alot to us.