Bullying behaviour is very common, and it's also very common for the "victim's" friends to go along with the bully because he or she is the alpha-dog.

I've saw quite a lot of it when I was in school. I've been the dumbass who stood by and did nothing as my circle of friends slowy ostracized a member of the group. And then I've been the one who was ostracized. The most extreme case was where one particularly evil-minded little snot thought it was the most hilarious thing to grab my boobs every time I walked by, and my friends told me I was over-reacting because they all secretly had a crush on him.

The Letter Writer isn't 13 years old anymore, and neither is her friend's husband, but even adults can get stuck in infantile group dynamics. She might be a humourless stick in the mud. Or she might be on the receiving end of some nasty group-think. They're so entertained by the guy's asshole act that they don't stop to think how cruel it is.