Fasterwolf has a point, call the police. print off the facebook messages, keep all the letters, and try to document all you can through eyewitnesses and evidence, then go to the police with it and file a harassment suit, or a restraining order or whatever.
if they can't do anything, while this might be more difficult, what your daughter can do is move. it doesn't have to be far, but a quick move a few blocks away will stop the woman from sending her mail. if push comes to shove, a new phone number. don't constantly run, but this would stop her. if it starts up again, seek further legal action.
magically speaking, a protection spell on your daughter, maybe a banishment spell on the woman. but this can backfire, so make a protection charm for your daughter [if she's fine with it] maybe a protection over her home and maybe a charm for the boyfriend, and that's it. if they don't want it, don't do it, and do not cast a love spell on those two, since that's messing with their life and will, it should not be done, and it runs the greatest risk of backfiring. just a protection should do.