Going forward, suggest they use a joint account for potential deductions, such as mortgage interest, property taxes, donations, etc., to make their options easier.
Not substantial authority, but a starting point with other links: https://www.irs.gov/faqs/itemized-deductions-standard-deduction/other-deduction-questions/other-deduction-questions
Then you'll want to dive deeper into your research, such as AnswerConnect with source material, Master Tax Guide, and the IRC.