It's hard- if the person is seriously involved with it or if they are considering it. Someone who has got deeply involved needs to figure out why. They already know what the repurcussions are etc. but they also need to realise: they are not in control. People who self harm never are and often it is a way to try and do this. I would also remind them it is selfish- to those who love them.
Those who consider it but are not really "into" it need to know it's a slippery slope. It starts off and it feels good, a relief but then you have to go further; cut deeper. It quickly gets out of control. Before they do anything rash they should talk to people, sort out their problems not try to forget about it through pain. It isn't easy if a person does it is addictive and when they stop they will have mood swings and cry a bit more. But their are ways to help:
Let the person realise, they ultimately have to wake up and see for themselves but highlight why you want them to stop. That's what I would do. :)
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