I'm dating this guy who has been in fake marriage for getting her green card about a half year. And he is trying to divorce this girl without getting her green card.

Ive heard faking marriage is a crime in the states. What happen if he gets cought?

I also want to know what happen to this girl after the divorce (she has only F1 visa which you can get when you marry) does this visa still work after the divorce or does she get deport?

F-1 visa is for foreign student, not spouse of US citizen.
If this is what he told you, he lied.

It mite be K1. I don't remember though

If he is married, wait for the divorce to date him.

Yes, it is a Federal ( not state) crime to fake a marriage.

it mite be K1. I don't remember tho

The K-1 visa is for a fiancee of a USC. If the K-1 visa holder doesn't marry that USC within 90 days; no way to get PR while in the US. Must leave the US.

They got married in 90days. But now he is trying to divoce, he sent immigration the letter that deny sponsor her . And her case got denied . In this case, is she okay to stay in the states currently?

She should leave as soon as reasonably possible. The USCIS sends the denial to the legal department to issue the NTA.

What if she stays there illegally ,then what happen to her?

The USCIS will start deportation. No way for her to get PR in the US without leaving except through the K-1 petitioner.

thanks for your reply, its really helpful. =)