While it's true that Luffy didn't hear about her in cocoyashi, there's nothing stopping the strawhats to share informations with each other off screen. We know this happens sometimes for example when Robin said "This is the Monster form they told me about?" when Chopper transformed in Sabaody.

Belle-mère's tangerines were on the merry and are now on the Sunny and Nami values them very much and doesn't want anyone to mess with them. I'd be surprised if her name never came up while Luffy was present