Father died - can I still file for him if I'm missing documents?

 

My elderly father passed away.  He had a live in girlfriend, he basically was her sugar daddy.  Our relationship was almost non-existent, however, he did leave us his 'shack' he lived in.   I was planning to file his final taxes and mark it as he's deceased, but we really won't get any paperwork since she's hording everything.   So we won't have any Social Security forms or from his retirement fund from where he worked.   She's a snake and I just don't want her stealing more from him than what she has already taken.  I don't care if there's a refund available, it won't be much, I just don't want her filing an gaining anything.  So I was hoping to just file a form, mark it deceased.  We don't know how they have filed in the past (joint or single) but they are NOT married.  Very complicated situation.