I think you're right not to comment on the post.
He may be starting a conversation with people she knows, but really you aren't that close in real life, and getting into a debate (no matter how offensive) isn't really the right vibe.
If you felt incredibly strongly, and felt that anywhere in the world you needed to counter that offensive sort of thing, then I think you could, but I'd suggest being non-personal and non-inflammatory. And be prepared for fallout.
(I disagree w/ Sparksals on the idea that it's wrong of you to call my opinion, or the posts on my wall, offensive. Some things just are offensive. Even if people believe them. Even if they're hotly contested.) The thing is, "offensive" *IS* an individual opinion. However, it's a loaded word, so I wouldn't consider that term "non-inflammatory.")