Matt I'm so sorry about what happened to your sister. 
Nobody is saying this man is actually a predator. But the fact is predators do exist and we are more aware of that now than we used to be, and that is a good thing surely.
How is it wrong for OP to tell her DD that she doesn't have to talk to anyone who makes her feel uncomfortable?
And why is nearly everyone on here so determined to believe this man is just a nice, friendly type and OP is a hysterical over-anxious mother when we just don't know? All those of you saying this was just an innocent, friendly comment - you weren't there! You have no idea how he said it, whether it was genuinely friendly or actually a bit creepy. OP only has her DD's word for it and clearly the DD found it a bit weird - so are we going to go back to telling children that what they feel doesn't matter?
It's the determination to disbelieve the OP's POV that is shocking on this thread; everyone piling in and slagging her off.
For my money, the kind of man that actually winds his window down to call out at a passing woman is not the genuinely friendly type. I thought it had kind of been agreed these days that that is low level harassment and again I'm shocked that so many people are denying that.
I take the point that women can say "cheer up love" too and men say it to other men on some occasions but I struggle to believe any man ever wound his window down to call out "cheer up love" to another man, or that women van drivers do it as a matter of course to anyone, male or female.
Really - why do people feel the need to have a go at the OP so much?