Where can I do if I know someone is planning a fraud marriage for green card ?

I know someone who is planning to get married in January 2017 so the girl get a green card . He is 51 years old and she is 28. She is from Mexico. I talked to her before . I just met her 2 months ago. That's the first time I heard or seen her. I know about this because I lived with him for 2 years. I'm still living with him at the moment . What she told me is that the way the met was because he help her parents get a house. She has only been here in his apartment twice . She doesn't stay for that long .i know he just wants to help but I think this is not the way to help by getting married if it's just for a green card