I wouldn't call it an amended return, I would do an original with filing status as single and attach a cover letter (also signed under penalty of perjury)  explaining what happened.  Will IRS be convinced they weren't married?  Well, at least they already see how much fraud he was capable of committing.  

"There are three stages of life: youth, maturity, and 'My, you're looking good!'"
Dwight D. Eisenhower