Reasons why a person could lose his green card?

Hello, I will like to know if a person that I know, could lose his green card, because he was married in his country Guatemala, when he obtained his green card he traveled to USA and he married somebody else here, all within the same 5 years, I know that is a bigamy, but his second wife is mental sick, she cant or wont do nothing, and the other wife, just stayed waiting for him to return or get divorce, and he did not nothing. What to do in this case? He lied to the immigration services, and also to two people