There's an obvious reason U.S. law contains a two-year "testing" period for new marriages and will give a foreign-born spouse of a U.S. citizen or permanent resident only a conditional green card, not a permanent one until their marriage has lasted for two years: Not all marriages last this long. And in some cases, the reason is that they weren't bona fide marriages in the first place, which gives U.S. immigration authorities a chance to root out marriage fraud. But "real" marriages fall apart sometimes, too. And the timing of the divorce becomes important if the foreign-born spouse still wishes to receive U.S. lawful permanent residence.
Here, we'll discuss a situation where the couple filed the I-751 petition to go from conditional to permanent residence jointly (though might have been separated at the time, but still on good terms), but then the U.S. petitioner files for divorce. It's less than idea timing, in the sense that, if the couple had gotten the divorce before the time came to file the I-751, the immigrant could have simply applied for a waiver of the joint filing requirement. But since it's too late for that, the question is, what next? Will U.S. Citizenship and Immigration Services (USCIS) deny the I-751 and place the immigrant in removal proceedings? Not necessarily.
First of all, the simple fact that your U.S. spouse filed for divorce, or that you were physically and/or legally separated at the time of filing the I-751 Petition to Remove Conditions on Residence is not enough for USCIS to automatically deny the I-751 petition.
USCIS might, at most, send a request for evidence (RFE) asking that you submit your final divorce decree. In such a case, you should perceive the RFE more as an opportunity than as an obstacle—because even if you failed to respond to the RFE by sending the requested document, but ample evidence on your record already showed that your marriage had been entered into in good faith, your petition should still be granted.
Once your divorce becomes final, however, you should quickly send a copy of your divorce decree to USCIS. It might be good (though not strictly required) to attach this document to a cover letter explicitly requesting that your initial joint petition be amended and changed into a divorce-based waiver petition.
In the alternative, the immigrant could file an entirely new I-751 petition. Indeed, there is no legal limit on the number of I-751 petitions a person may file—although practically speaking, submitting multiple filings could confuse U.S. immigration officers and, therefore, delay the adjudication of the case.
In either situation, the immigrant should consider also providing an explanation of the circumstances of the divorce, with evidence of your attempts to save your marriage (marriage counsel records, for example), in order to strengthen your good faith marriage claim. Be prepared to elaborate further on these points at a future interview.
Given that your situation is unusual, consider hiring an experienced immigration attorney. The attorney can analyze the facts of your case and spot any potential red flags that might cause USCIS to believe your marriage wasn't bona fide in the first place, gather supporting documents and prepare the paperwork and a cover letter that summarizes why your case deserves a favorable decision, and monitor progress toward approval.