I masterbated frequently as a child, and I remember my mother telling me not to do it in front of people and feeling all ashamed, etc *I'm not saying your girl feels this way, but it was always a source of shame for me*. As someone who was in her position, I encourage you to keep helping her not feel ashamed about it.. the shame.. it's horrible, trust me. I recall up into my early twenties (wasn't frequent by then but still) feeling a level of guilt when I was "done" that would sometimes make me feel suicidal. I felt THAT disgusting.
I'm also not saying this is in relation to your girl, but was she ever abused or anything? I only ask this because I was molested and also started masterbating at a very early age. I don't know if there is a connection or not.