would she have to tell her parents about this guy in order to block numbers? if that's the sticking point, could she say something like "this girl Jane Doe keeps calling for John Smith, no matter how many times I explain this isn't his number, and I'm sick of dealing with her"? i don't know if lying to her parents is the ideal situation, but it's probably better than continuing to get his harassing texts.

also, i don't think you're overreacting.