HELP!How to help my 17yr old sister cope with the death of her boyfriend of 3 years?

My sisters boyfriend was on his way pick her up two days ago, an his way something went wrong and was shot to death twice in the head and once on the side. She feels guilty because she was mad at him because she had thought he stood her up and was not answering. She is heartbroken, will not sleep, eat or talk, just cries hysterically. Does anybody have any advice on how to talk to her to help her cope? Im afraid she might harm her self if she is left alone.