First, I just don't see Reiji as purely evil any more. I really think he will want to help her.
Second -- does she even need help? I don't know. I think psychologists would say that what a person does about sex tends to be OK if it doesn't interfere with the rest of their lives too much. Rin is right on the edge of that, but she has always been ridiculously sexualized. We tend to forget -- in Chapter 1, she was joking with Kuro about porn stars. I could look up the reference. How would she even know the names of any porn stars?
http://www.mangafox.com/manga/kodomo...1/c001/23.html
Masturbation is a tough topic because according to lots of people it is something that never happens, or should never happen. And yet it is certainly close to universal among males and quite common among females. Probably the young boys in Rin's class are more likely to "wear themselves out" at that age than Rin is. But that is not what we are being shown here...