Well, there's no need for excuses either way, and this is the explanation for why she has this pronunciation. Pronouncing sounds that aren't part of the phoneme inventory that they are familiar with isn't as easy for all learners, especially learners who learned English via grammar translation method (http://en.wikipedia.org/wiki/Language_education#The_grammar_translation_method) by teachers who aren't able to pronounce THs either and don't care for pronunciation in general. And after twenty-plus years of using a certain pronunciation without ever being corrected, it's veeery hard to change things for both teachers and learners.