How do I report someone who claimed to marry for love but soon as he got the green card he left the wife

You read this site and report the person: USCIS Home Page (http://www.uscis.gov/portal/site/uscis).

Why would you get involved?

A large number of these reports are filed. Very few are acted upon BECAUSE it is entirely possible two people marry, one decides he/she wants out of the marriage and the green card has little to do with it. You would have to PROVE the US citizen was not a knowing part of the arrangement AND that the non-citizen committed fraud.

It's hard to do.