Not reading all of that, but if someone that has dealt with people that get either the 2-year or the 10-year green cards, they are probably certain things that it says will put you in violation. Not sure if he did anything to violate those things though.
Why post then if you're not going to bother to inform yourself? You admit you didn't read the details, but you're already assuming he must have violated something? That's exactly the kind of lazy thinking that lets the government get away with this.
He was detained by DHS agents who didn't even know his actual status. First, they claimed his student visa was revoked, except he had a Green Card. Then, they suddenly decided that that was revoked too. There was no due process, no legal explanation, and now he's disappeared into ICE's system.
Green Card holders have First Amendment rights, too. And there's no law against protesting. There's no justification for this. This is blatant political retaliation.
You should be opposed to this no matter where you stand.