Well, the dental fricatives don't exist in the German phoneme inventory, so many beginners substitute all "th"s with what they consider to be most similar - which is unfortunately leaves them with alveolar fricatives and native speakers as well as advanced learners with sore ears.

In the olden days, actually speaking English wasn't a very important learning objective and English lessons mostly consisted of translating passages from English to German in the vein of our classics courses, so this particular student sort of has an excuse because her last English lessons was in the early eighties, and she's bound to have been taught by these teachers. Still, considering that she apparently swears that she watches the news in English every day and still can't tell a difference between the pronunciation of these words... yeah. I don't know what's up with that.