I write this out of personal experience. I am not a mental health proffessional, but I am an SIer.
I know that the SIer you know may be hurting you deeply by their actions. I know this may make you feel angry and/or concerned for their health. But know these things before you decide to make any decisions. Know how we feel, and understand how SIing works. Study it... there's many sites out there that give professional and personal advice and information about SIing.
An SIer, first of all, usually keeps their SIing secret. If an SIer has entrusted you with the knowledge that they SI, DON'T VIOLATE THAT TRUST! SIers keep their SIing secret because of shame, fear of others anger, fear of others trying to take their habit away from them, and fear of being forced into things. Don't do ANY of these! Even though an SIer fears these things, don't think that using them once they have confided in you will make them stop. Not only will it make them not trust you, but will force them to become more secretitive and more creative in their efforts. Also, don't demand to know why they do it. Research the reasons other SIers give if you need to know. The SIer you know probably will simply clam up and won't tell you, simply because they think you'll think they're crazy or psychotic.
What you can do: Be supportive. Don't bring the SIing up unless they do, but simply notice if they seem upset and ask them why, give them a hug once in a while for no reason, ask them how their day went. Make it genuine, and don't overdo it. They may seem standoffish at first and uncommunicative, but if you keep being supportive of them they'll eventually warm up. Don't bring up the subject of therepy. Most SIers curl their lips up in disgust of therepy, especially if they've had bad experiences with it, and this usually just makes them back off more. One thing you can do though is make sure they're being safe about their SIing. For example, if they're a cutter, simply ask them one day about if they sterilize their cutting device and dress the wounds afterwards. Hear their answer, and if they say no, encourage them to do so, and leave it at that. It's a way of showing you care without pushing them to stop. Know that nothing, NOTHING can make them stop until they decide to stop. You just want them to feel comfortable and trusting enough of you to come to you when they want to stop for help and support. Sometimes a good, supportive, caring person is all an SIer needs to make them want to stop. Most SIers have given up on people and themselves. They need someone who they can trust, and they need someone who can tell them they're not crazy or psychotic. Also know that this is only the first step for them... wanting to quit.... some SIers also need therepy to fully heal once they decide to quit.
