You should think of involving the police rather than look for an it expert for such problem. If if you think it is that serious, then you should seek the help of the authorities. They have experts there with respect to such issues, however, I don't know how much their level of expertise is.
I, however, don't think such threat is a real one. Someone who really intends to inflict harm on another, won't advertise such intention, only to spook and scare your friend to be cautious. Such reaction from your friend will be detrimental to such person if he / she really intends to hurt / harm your friend.
My advice is tell your friend to forget about such problems and show that person that he is not afraid and affected. The threat, in my opinion, will go on, as long as he gets the "attention" and "reaction" he is so yearning for from your friend.