smile and say hi
Yes that could work!
He must be very dumb not to respond.
But I know even the boldest men would say: Hello!...and then wonder, now what?
(Had to push this friend to go move on, say something more, despite he acted like macho in all men private activities)
I mean don't underestimate the guy he might just be a bit reluctant depending on his background or something.
Now if the girl is really interested, and does not want him to feel pressured or make him feel she is somehow "too bold", "experienced" or even "slutty", she could still go on and talk usual or casual stuff for a few minutes and go on.. Allow him to move further afterward.