You could not believe me (as a random person on the web), but your phrasing implies that you also consider this somehow beyond the realm of possibility. It happened in Athens, if that matters.
>do you seriously feel that this actually works in changing the attitude of a society
Yeah, where's the surprise? Changes in demographics, like an increased presence of people with more conservative values in an area, can, and do, change the attitudes of a society, just like immigration can influence cuisine, music, religion, and other cultural aspects.
I contest believing that young people going to beach in France or Italy would give even passing thought to what these people think or want, because it's completly outside the realm of my experience of the people that I've ever known to go to the beach, ever. A vastly more likely consideration is avoiding unwanted attention from people that approve of mostly undressed girls.
I don't think that you understand (I don't know, maybe you live here?) how socially -isolated- immigrants are.
On the beach, no, but it can be a factor on how people dress to go outside on the city, or to avoid certain streets, and so on.
And she was not even that provocatively dressed (way less than the average working class girl -- what english would call a "chav" -- in her late teens going out for a drink on friday/saturday)