I am not sure that the fact that the recipient of the prank call was in another state doesn't create a bit of an issue for you or your friend. The California Penal Code might not control. Perhaps the fact that the victim, if they are in fact the victim, was in another state might mean that that state's law would be invoked rather than California's. If she were to complain to a prosecutor, it seems doubtful to me that she would complain to a California district attorney rather than a prosecutor in her own local jurisdiction. Also, it is quite conceivable to me that there might be federal laws that address this issue that I am unaware of.

On the other hand, it seems very unlikely to me that someone who did not actually receive a message but merely was harassed by silence would want to aggressively pursue the caller after all this time. Perhaps the best thing to come out of all of this is the remorse that seems to have set in now. Such behavior is worse than juvenile. It is quite probably actionable civilly and/or criminally. What will probably come out of this is the realization that such behavior should never be repeated.

Good luck to your friend.