That is a rather serious question and one for which I am not sure I truly have an answer. But I shall do my best.

Firstly, I believe that my response must depend on the circumstances involved. If this was something wherein we are not living together, perhaps even a long distance relationship, then my actions will be limited.

In such a case it would be best to talk with her, to counsel her and to listen. All I could do is to be there when she needs me, a text away, a phone call at times, or however, she needs me even if we could not be physical available. Perhaps, as time passes, I could be available when she is having her worst moments to at least be an ear as she nears causing harm and steering her away from such actions.

In time, perhaps, some alternatives to the harm could be made available. If we can determine, together, the source of the harm, then maybe a valid method of redirection to those emotions could be started. 

Now, if it was a situation where we lived together then all of the above would still be true. But, if possible I would wrap her in my arms, hold her as tightly as I could and keep her from harming herself. This may not be the best solution, but it is one thing I would try. Predominantly though I would listen to her, let her vent and scream as needed and do my utmost not to let her harm her body or mind.

In summation I would listen, try to help her not harm herself as best I could (physically if possible or by speaking with mutual friends, if applicable), and perhaps help her to seek out counsel if such things would be the best course of action.