In the end, none of those responses actually seemed like enough. Instead, I decided to respond in a blog post, and let others make up their own minds. If you know me, you know that generally I tend to use FB for social interaction and not for political/social agendas (other than my passions, autism and education). This bothered me enough that I'm going to break that rule. Right now.
Here is the photo and text of the interaction, with names and profile photos kindly redacted:
Personally, I think all of these people should be ashamed of themselves. They are the reason that some children think it's okay to call other kids "fat" or "stupid" on the playground. I'm sure they thought it was just a joke, you know, someone to poke fun of on a boring Saturday night. Two fat women they don't know, just walking on the cattle...er, people...mover. What does it matter if they are someone's mother, grandmother, daughter? What's not funny about fat women walking around?
I didn't find it funny.