Hidden facebook stalker?

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Ok, going to try to explain this without to much back story. My mom's new hubby has an ex with kids. One of her kids keeps trying to add my mom as a friend but when my mom try's to block them it says "no response". When she asked me to try and it said the same. I've looked in to it a bit and my best guess is they have turned their Privacy Settings all the way up and/or set them to just block her, but this person keeps adding her. The only answer I found is to contact facebook but I've tried to talk with them before and long story short that was a joke. Can anyone (maybe a facebook tech...haha) tell me how she can stop this annoyance? Isn't the whole point of Privacy Settings to keep your facebook experience fun?