Someone got married to a U. S. citizen just to obtain his green card and she did not know and when she found out she asked for divorce. He was cheating on her and even had someone else pregnant.

First there are plenty of people who marry, cheat and get others pregnant. The issue is what proof do you have, do you have it in writing, or is it just an opinion. What proof, that would stand up in court ( or a hearing do you have)

Next what is your relationship to it all.

I want to know how to report a person that signed me signature to extend his green card?

I wanna know how to report a person that signed my signature to extend his greencard?
I need