I don't know if "not habitable" are the right words for those parts of New Orleans you are refering to. I helped with cleanup in the Ninth Ward a year after Katrina. While parts were total devastation, even a year later, if you bulldozed the entire area and put up new development (or, in certain areas we worked, if you simply refurbished certain areas that were salvageable), you could live there. What I'm getting from my friend's Facebook post is, if he's not overexaggerating, this storm is such that it will alter the greater Houston area in such a way that certain areas will never be habitable again regardless of a cleanup. Keep in mind the flooding in Houston right now is as bad as the flooding ever was in New Orleans after Katrina - and they're supposed to get another 50 inches on top of that.