This is tricky.  One one hand you have the Statute of Frauds, which requires such items to be in writing and on the other hand you have the issue of performance. Your mother performed, and initially intended to be on the note.She should consult with her estate attorney to determine what the best situation is for her but if she signed initial paperwork during the refinance and it was simply a scrivener's error not having her on it, then she is most likely out of luck.