There are two mechanisms which come into play in the causation of OCD. These can be grouped into biological cause and psychological cause.
As far as the biological aspect is concerned, to cut the long story short, a particular brain circuit comes into play. This is the orbitofrontal cortex and the basal ganglia circuits that are implicated. The signal in the circuit runs over and over. This leads to repetitive thoughts.
And the psychological aspect goes back to the initial years of life. Some people have suggested a hypothesis that a strict toilet training eventually predisposes a person to OCD.
Prevention is not the issue now. She has already got it. Initially she will need medicines, as I mentioned in the previous post. Along with the medicines, she can take behavior therapy. When thoughts come again and again, some patients use a rubber band around their wrists, and snap it every time the thought comes. This method will help control the obsessive thoughts. There are many more such methods.
Let her be assessed by a Psychiatrist. He will be able to judge if she can be given behavior therapy only.
Here is a rather good website that discusses OCD, causes, treatments, and it has some other links, too:
There is also an Anxiety Community at this website, too, where you can find a few posts about natural treatments for anxiety and also dealing with OCD.
OCD can be helped somewhat in several natural ways. The B vitamins found in protein foods are good for keeping a person calm. So is regular exercise, enuff to break a sweat, especially the rhythm of walking, biking, running, and dancing. Also it is important to balance the stresses of life with pleasant and relaxing pasttimes. Likewise, taking very warm baths and getting a massage can unwind tension. I'm not a big fan of herbal treatments because they can have side effects like drugs do.
Most importantly, tho, whatever you're doing obsessively, like washing hands, just force yourself to go a whole day without doing that at all. Your mind will realize nothing unusual is going to happen if you don't wash your hands, and the obsession will lessen. Another good practice is, whenever you get caught up in your compulsion, stop and sit for a few minutes and check your pulse, breathe deeply, focus your eyes on a distant object, until you feel your body relax and heart rate go down.