listen to them. really listen.
if they've tried talking to about it, then they probably are asking for your help already. if they haven't, then they may be blaming themselves for the things that are going wrong, or doing it for attention, but still talk to them.
also, dont tell anyone about it. its probably best if fewer people know. 'cuz if they have talked to you about it, and you tell people, they may keep cutting, and not tell you the next time. respect the fact that they want help.
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