I have no idea what JWs teach about the manger, I always thought they claim the cross was really either a tree or a wooden stake.
I did see a documentary, probably on DSC or PBS or someplace not too long ago, anyway a secular production, not religious like EWTN, about Christmas, the nativity story, Bethlehem, shepherds etc. and it showed examples of stone mangers fixed in place. These were in the type of dwelling, apparently common in the Holy Land, that are made out of caves or carved into a hillside, where the family sleeps on a platform above the animals. There were stone mangers for straw or other feed, and stone water troughs, each somewhat different in size and shape. So I suppose it is not too far-fetched that the manger in which the Christ Child was laid could have been stone, but unless it is shown marble occurs in the vicinity naturally it would be hard to prove that claim. but it is something that could probably be supported with historic and archeological evidence if you think it matters.
I too am an ageing female, who finds it increasingly difficult to find energy to debate non-essentials. Real apologetics is exhausting enough. After that slam, I will go on to say what I was going to originally, but had second thoughts about in the name of charity. Heck, as an ageing female I can say anything I please. I was going to observe that what I find far more troubling about the exchange recorded in OP was the fact that one person offered an observation about details in the life of Christ with a devotional intent, and another person jumped on his comment, riding right over the intention of the comment. If this is a mother-child confrontation, it speaks volumes about a relationship sadly in need of healing, which we can certainly pray for.