You wrote that he had rented it for 20 years, so you need to know the fair market value at the time of conversion to rental use for depreciation purposes. His gain, however, appears to be substantial based upon the purchase price.
Obviously, he has owned it for far longer than the rental period. Regardless, if he started renting it in 2005 or 2006, his accumulated depreciation is substantial, and he will be subject to Section 1250 (i.e., recovery of accumulated depreciation deductions).
