My ex-girlfriend used to cut herself and was very shy and insecure about it all.
Most people didn't know she did it - it showed up more when she had a suntan.
My ex-housemate also used to cut herself and was also shy and didn't like to talk about it with most people.
I remember a particularly enjoyable evening when she came back pissed as a fart, cut herself and it bled everywhere. Calling an ambulance while she's screaming at the foot of the stairs and bleeding everywhere is
Anyway - based on my somewhat limited experience I wouldn't talk to her about it. It isn't your business and she won't appreciate it.
Noone "knows" why people cut themselves. It's supposedly a way of getting stress of out your system (by diverting the pain, so to speak). It can also be a cry for help, or some form of attention seeking. Some people supposedly just like the adrenaline rush you get with the pain of it.
Anyway - this is often caused by more deep-rooted problems (depression or more severe mental disorders). In which case it would be a good idea to go to a doctor.
But she has to accept that it isn't a good thing first. And "showing it off in the ladies" doesn't sound like she knows that. It sounds like she's a ****ing idiot to me.
But again - it isn't your concern or business, so don't take a leading hand in it. It'll lead to animosity and bad feeling. And you won't be helping anything.