Dad - When all else fails, read the IRS Instructions https://www.irs.gov/pub/irs-pdf/i1040sca.pdf page A-8
"Limit for loan proceeds not used to buy, build, or substantially improve your home. You can only deduct home mortgage interest to the extent that the loan proceeds from your home mortgage are used to buy, build, or substantially improve the home securing the loan ("qualifying debt"). Make sure to check the box on line 8 if you had one or more home mortgages in 2018 with an outstanding balance and you didn't use all of the loan proceeds to buy, build, or substantially improve the home. The only exception to this limit is for loans taken out on or before October 13, 1987; the loan proceeds for these loans are treated as having been used to buy, build, or substantially improve the home. See Pub. 936 for more information about loans taken out on or before October 13, 1987."
Sorry Dad, but you need a new barber