He avoids paying taxes on the sale of stocks to finance the home.
Mortgage interest is tax deductible.
His stocks will almost certainly yield more than the interest rate on the mortgage.
He's getting a sweetheart deal from the bank.
For him, selling stock is a PITA because he has special class of stock that grant him voting rights far beyond what a normal share is worth. These special shares are granted to him by the board of directors. So selling 1 share means losing like 1000 votes (they convert back to 1:1 upon sale), meaning he needs to be careful when liquidating assets. By waiting five years, he can use vested options to pay the mortgage, rather than selling his special class stock.