I'm quite surprised by the attitude of the Cabin Crew. Surely what she could do is threaten to have the police waiting for him on arrival for failure to obey her/the pilot's lawful orders?
I have no idea whether using a mobile in flight is actually dangerous. But people who have thought longer and harder about it than me consider it may be. That's quite enough for me. Even if the chance is absolutely remote, why should one d!ckhead decide he knows better?
IMHO it was the flight attendant's job to enforce this rule - I wouldn't be impressed if they simply didn't bother (in the absence of other extenuating circumstances, eg. drunk, aggressive passenger who might constitute a more immediate threat if told to turn the phone off, then it may be a question of balancing one risk against another).