Knowing Luffy, he'd probably stare blankly picking his nose with his normal clueless expression on his face.
It would probably take Luffy a few seconds before he realizes who she is (he's slow like that), and with the same expression, he'd ask something like: "Hancock, did you trip in flour and get a cherry stuck to your face?"
This of course, would make her burst out laughing! "Oh Luffy-Kun! You always say the darnedest things - that's what I love about you!"