For some reason, this thread keeps reminding me of the one about A, B and C, where one of the men keeps bringing someone besides his wife to their gathering, and it's making people uncomfortable.
Sure, there's a difference between posting photos on FB and showing up with someone you're not married to, but...what actually is the difference? How does it get articulated? This is why I totally disagree with people saying it's not about etiquette. Of course it is. The issue isn't about the morality of their relationship; it's about how to handle it vis a vis others in a way that's least likely to be hurtful. In OPs scenario, sure the husband, the kids, his family and friends can block or hide, but by the time they know to do this, the damage has been done.
The way I see it, it becomes the business of others when it gets presented to them. Just a theoretical, but if you (general) went to a party and a married man showed up with someone besides his wife, and his wife is a fairly good friend of yours...does it become your business? Is the guy at this party on solid ground because it's nobody's business what his marriage is like? Sure, the analogy isn't perfect, but some basic principles still apply. It's very possible people close to the husband found this distasteful or upsetting. In that case, how can it be good etiquette?
I know my viewpoint is a minority one, but I still find the differing POV's pretty fascinating!