A few observations....The ICE will assume all marriages to Foreign nationals are fraudulent and you have to prove they are real in order to gain LPR status. However, for them to come out right and "accuse" them of fraud, they must have some pretty concrete evidence.
Is the marriage fraudulent?
"In order to stay in the U.S. he decided to get married"
This is Fraud. Were they involved in a normal relationship before the marriage? I know you said they were committed after, but if they actually said this, that would be a reason to doubt the marriage and be accused of fraud.
Generally speaking , if a couple enters into a marriage for the right reasons, and it does not work out, the LPR can still gain permanant residency, it will just be harder to prove that it was real.
He should continue to pursue this option, hopefully the ex will be cooperative and help explain how it was real, but it soured. He needs to show TONS of evidence of how the marriage was real, while it lasted. Involvement with the in-laws will help a little, but would not be a complete vindication.
Ultimately, if they do not believe him, he will be deported. If that happens, he will not be eligible to return to the US legally

