I dont think this is true. I came through i864, went through divorce. After my divorce was done, I filed for support based on i864 at the Federal court, ofcourse was represented by an attorney but I had done all the homework related to evidences, proofs of income and defenses etc, it was a lot of work before I approached attorney to initiate this. My ex had two top attorneys but they knew it was a losing game and finally settled for what we asked.
The fiduciary duty to support will be governed by the state family law, not the immigration laws or any obligation to support associated with it. In this case as the sponsor withdrew the petition and thus no green card was issued, there is no enforceable contract, simply because the contract did not ripen. The contract says if the immigrant is given a green card based on the marriage then the sponsor has an obligation to support her at a certain % of poverty level. Since the green card was never issued, even though the sponsor would have submitted a signed contract to USCIS , and a copy of which the wife can procure via FOIA, the defense would be there was no green card issued and thus the contracts conditions were never met. Depending on the length of marriage and the common property generated during that time and the states family laws the husband might be on the hook for some short term support and it largely depends on the income and standard of living during marriage. The wife has no other option to gain a green card via this marriage or any other option. VAWA would require proofs such as reliable testimony or police/hospital/social worker reports etc. Else everyone could use that route to get a GC. She could go back on student visa or get remarried and gain green card via that.