Exwife is now a dual citizen by filing with INS as an abused spouse
Something must have been filed at some point (of which we do not know when) if she is now a US citizen.
Bali...as pro said the timeline makes that impossible....at least the timeline as we know it. It takes 5 years to gain citizenship unless you were sponsored by a spouse and then its 3 years, and he did clearly state that he did NOT sponsor her.
What she probably got, when they separated, was a green card based on being an abused spouse, and that doesn't necessarily give you permission to work. It usually does, but not always.