Accountable Plans are generally best to do monthly, but need to be at least every two months.

https://www.irs.gov/publications/p463#en_US_2022_publink1000266091

The taxpayer should receive some sort of 'bill' for their mortage that shows the amount of principal paid and the amount of interest paid.  Also, they likely have an amortization schedule showing the monthly amounts somewhere, or you could re-create an estimated one.