I actually had this problem once before, where a certain person kept calling my number despite me telling him that he has the wrong number. I would voice out the same like the others had done which would be to try calling the service provider. I know some mobile phones can block calls from a certain number as well, but sadly, that does not apply to the iPhone for some reason.
I truly understand how your friend feels though, and hope she would know that she has friends and people who would support her when it comes to this situation. :/ It's just disturbing and can be so nerve-wrecking; especially when the phone keeps ringing like almost every time. Fortunately, the person stopped after I ignore the calls and I actually did sent a threatening text that if he did not stop, I would call the authorities.