You cannot petition an in-law. I am therefore going to assume you are talking about the Affidavit of Support. If so, there is no way to revoke it. See link below.
Additionally, if they had a baby together, then she obviously did not marry your son just for the green card. I just do not understand why people assume that because a marriage fails it was just for the green card.
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You don't have it within your power to revoke anything. If you signed a joint affidavit and she got the green card based on this, you are obligated. See link for when the obligation runs out.
If you have actual evidence of fraud, you can supply it to USCIS or ICE and see if they'll do anything - but just her leaving isn't proof of fraud.
She would at first have likely gotten a conditional green card good for just two years - would have then needed to "remove conditions" to get a ten year card or the status expired. Not clear if this was done here.
Is she has a green card, it is unlikely you can withdraw your I-864 affidavit of support
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You cannot revoke her green card. You are still under obligation under the Affidavit of support but only if she collects a government benefit. Assuming your son is supporting his child and divorce/separation issues have been resolved, that is not likely to occur. The focus should be on the domestic relations issue, not immigration.
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