I think you're asking about the estate income tax return, Form 1041.  Not a Form 706 estate tax return.  And was your client a US citizen?  If so, probably not wealthy enough to worry about estate taxes.  (For resident aliens there are special rules, which I would have to look up.)  

Yes, the estate can elect to be taxed on a fiscal year basis, ending in September 2025 or any month before that.  Does the estate have income?  Will it be fully administered by September?  Someone has to make decisions here, and you're not the one in charge.  But you can advise them.  Otherwise, did the estate have any income from date of death through December 31? If a calendar-year 1041 is required, you have a week to prepare it, or file an extension.