I'm sorry your cousin has to deal with that.
If he was baptized at a young age, and claims he did not understand what he was getting into subsequently being DF'd at 18, it is POSSIBLE, for him to protest his shunning with the local brothers and branch.
In particular the new elders book that was leaked makes allowances for something like "time served" in certain situations.
"I was baptized a minor....I should not have been. My family believes in shunning me continuously over this technicality. As a result both they and I suffer. This behavior in and of itself has made me not want to connect with the religion of my youth. I am neither an "evildoer" nor "sinning". Is this sort of long term family excommunication really the point of this doctrine to begin with?"
Now this would take some work and unpleasantness to be sure.....but it might be away out of this at the local level.
I certainly hope he does not allow his family to see his child or wife while they shun him. Talking about sending your kid the wrong message. Though I would never judge the guy either way. That is a tough one.