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I am sorry to hear about this situation.
If she has already received an unconditional (permanent) greencard based on the marriage, then his letters to USCIS or its sister agency NVC would not be acted upon. At this point, USCIS has confirmed the marriage as valid and if so, it is too late to really do anything unless he gets an annulment of the marriage (not a divorce).
If she currently has a two year conditional greencard which has yet to have become unconditional, then USCIS may decide to disallow her to make it unconditional if it feels that her marriage was for convenience, and not love. If so, then her application to make the greencard unconditional (I-751) may be refused. In this situation then, his letters to USCIS may very much make a difference.
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