Try to talk to the girl first to try to get her to talk to her mother, then talk the mother - but be careful, although she may be posting photos online that doesn't mean she wants to be 'confronted' on what she's doing so she may react badly. From there hopefully the mother will get her daughter help in the form of a therapist.
Also something a lot of parents and friends of self-harming people do is unintentionally make them feel bad for cutting, or they will try to get them to promise to stop for them...this is the worst possible thing anyone can do. Self-harm is a coping mechanism, take away a persons coping mechanism either they will find another even more destructive coping method such as drugs or sexual promiscuity, or they will simply not be able to cope. I've seen multiple cases where teens have tried so long not to let down other people that this plus all the upset they're experiencing makes them snap and they end-up hurting themselves far more than they would normally, including suicide or accidental death.
The girl needs therapy - they determine why she doesn't have normal coping skills, what is triggering her self-harm (e.g. if she's depressed, suffering abuse at home or bullying at school, etc.), how to avoid triggers, and give her more healthy coping strategies.
Make sure your dd can talk to you about it. Supporting a friend in distress can be very upsetting and your dd needs to be able to vent and dump some of it onto you. Don't mean she should repeat everything to you, but that if she has concerns she's not on her own.
DDs friend sounds very unhappy. The fact that she's posted photos online suggests to me that she wants help but doesn't really know how to ask for it. Can your DD encourage her to speak to someone? Whether a school counsellor, gp, parent or Samaritans.
My daughter has come home from school in a state tonight as she has figured out that her good friend is cutting herself... She has found an instagram account of her friends with images of her legs where she hs cut and it appears to be out of control.. I know the girl well and suspect she has body image issues and self esteem issues - mum is lovely - but I have no idea what to do here..... I could talk to the girl myself - we know her well enough and I am very fond of her...... Can anyone give me any idea what to do ?