I may disagree with Carl. There is a legal concept called equitable ownership - the other party does not need to be on the title or mortgage. see the link. https://www.taxcpe.com/blogs/news/equitable-ownership-and-mortgage-deductions
we are not lawyers and this is something that may require a legal resolution that should be in writing. possibly finding yourself in court and needing a lawyer gets to be expensive. there are also the tax issues of whom should be reporting what. This presents significant current and future issues for both party's taxes.
