Citizenship

I do have a friend who just got rfe on his pending n400 so the issue is on his visa application 6 years ago there was error in his application done by the preparer cus he hired a traveling agent to do it for him but then they have it there that he was married with a female name that doesn’t even exist to him and then he wasn’t even married at all . well he didn’t know that and he signed the application got his visa get here and married and he got his 2 and 10 years visa without no issues but then during when he was filing for his citizenship during the interview they lady brought up the error on the visa application years ago which she told the officer . now they sent him a intent of denial requesting more document and information about the marriage on his visa application . so now he is trying to know that even if worst come to worst and they denied his n400 he is scared maybe they can come after his green card and if he is protected under the 8 U S C section 1256a that says uscis can’t revoke a person green card after 5 years of them being a legal residence . pls drop ur comment