It is the extreme of selfishness. You don't love the person but you only love the status and money. If you know such a person who is going to marry some of your friend with these intentions then you can simply tell them the truth. The rest they will find by judging the person's actions and behavior. Lie has no legs and truth has all the power of the world.
I know a woman that marry to this man and pay him for the green card and as soon she became a usa one month or 2 later she marry this man from peru only because he will pay her the home and bills. Every body know it.
It will not do any good to blow the whistle as the regime in Washington pretty much ignores illegal immigration and the desires of the citizenry. What ever you say would just be lost in the bureaucracy.