I dont think DHS will do much if you write in, if the USC does marry this person, he/she is obligated to support her for the next 10 years, (40 quarters) if you definately think this is marriage fraud tell the person to read the instructions of the I-864 very carefully. Also the INS are not that stupid, they need to supply evidence of a bonifide marriage, and once they get to Interview stage, if the Immigration officer "smells a rat" they will be questioned thoroughly.

