One of the people I know just received his permanent green card, however I know that he was involved in the communist party, numerous communist totalitarian clubs and continues to be involved and spends great amount of time on communist propaganda (on social networks).
How do I report him to the immigration and even if I do what would be the consequences for him?

Report him for what, belonging to a legal political party.

Where have you lived for 30 years, it is completely OK to be a communist and be a US citizen

Or even to be the president.

www.cpusa.org

They have been part of American politics since 1919. They even have people that run for many officers at state level in some states.