Educate them, send them a link to BB, encourage them to get some books on the topic? Something? Without BB and other gentle parents telling me it was ok to hold your baby, to pick them up when they cry, to not have baby in a strict routine from day dot etc I probably would have been just like them.

ETA: I think though it would be important to do this in a gentle, non critical way as much as you possibly can.. Maybe a "Ooooh I thought that was the only way too, but I have heard fantastic success also with these gentler methods, maybe have a peek at them before you try CC?"

FWIW My DS slept through from 6 weeks too...Being demand fed and without CC. Some babies are just like that! There's no wrong or right about it.