Immigration Law Questions? Ask an Immigration Lawyer.
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I'm sorry to hear about your situation. You can report immigration fraud by going to the following website: http://thelawdictionary.org/article/how-to-report-immigration-marriage-fraud/
You don't have to be married to the person that you're reporting (that would significantly undermine the fraud reporting mechanism if that were a requirement). You do need to know, however, that they'll still look at this suspiciously. Often "jilted lovers" (someone who is cheated on, or going through a separation or divorce) will report their former significant other. In these situations there could be less than the amount of evidence that the US Citizenship and Immigration Services (USCIS) would like to have to pursue prosecution.
It's a good thing that you heard her once mention that a friend asked her to do this favor for her, but it might not be enough. That's "hearsay" and often is not allowed for the purposes of showing conformance with that request. But it might still be enough for them to look more carefully at any application filed with USCIS and investigate it a bit more thoroughly for fraud.
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