She may start the divorce process now. It doesn't matter who starts it. She also does not have to wait until her conditional green card expires. She can either inform USCIS to amend her I-751 to change it to a waiver or she can file a new I-751 in waiver form. Here is some information on that:
Focus on where it says, "If your marriage was terminated other than by the death of your spouse, you must submit the following:"
She will need to submit evidence similar to the evidence that she submitted at her original marriage interview. She must prove that even though she divorced, her marriage was not entered into solely for the purpose of obtaining an immigration benefit.
Here is a list of things that I like to give my clients and I know that she may not have all of these things, but the more she has, the better it is.
A letter where she describes, in a very detailed fashion, the circumstances in which she met her spouse, the reasons she married, and the feelings and emotions that she had or still has towards him, and why.
If they have a child or children together, the birth certificate for each child.
Wedding pictures and pictures of other moments when she and her spouse, and other members of each family and friends have been together.
Very detailed letters from people who know them and who are witnesses that their marriage was valid, who were at their wedding, or who knew them as a married couple.
Love letters or cards that they received from their spouse while in the relationship.
Letters from people addressed to both of them, or in which the person who wrote the letter refers to both them as a couple.
Any types of documents that have both their names on them that show that they bought a car, a house, furniture, or anything else together.
Any rental or lease agreements for their home or apartment with both of their names on it or a letter from the building manager or owner proving that they lived together.
Bank or financial statements that show that both had or have a savings or checking account together.
Any insurance documents that show that they were or still are covered by their insurance plan, or that his plan covers or covered her.
Bills, such as electricity, water, heat, cable TV, phone, or others that show both their names.
Jointly-filed Income tax papers (both their names).
An identification card that shows that his spouse's was using his last name.
Membership cards for video clubs, grocery stores or similar businesses, that show joint membership.
Any other documents.
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