The story is unlikely to be true, but there's a slim chance that it could be these German serial killers in Greece and the story's been muddled through the multiple layers of retelling it took to get to us (from the professor who experienced it, to the professor's student, to the student's brother, who told us). The timeline roughly matches up, and the lack of free press at the time could also explain a lack of evidence specifically about the professor's experience.

If he's close to 80 and did this in his 20s that'd be in the 1960s.

This is before chartered tourism was a thing and probably during the kingdom before the following dictatorship from the Junta coup in 1967.

That's a pretty wild place to be as a Swedish hippie. Especially since the hippies didn't really happen until the end of the 60s. I also find it unlikely that the police would have any interest in pursuing anything from that time or even have records of their visas.

I'm not saying it's bogus, but for a history teacher, he should probably get the year right and admit that it happened in the 1990s when he was closer to 50 years old.