Caliber and age are important in judging value, as older guns bring more. A pre-WWII gun in true 100% will bring $450-500; a post war gun around $100 less. Oddly, in spite of the magic Colt name, there is only minimal collector interest in the swing cylinder DA's, though ones like you describe get most of their value as collectors items. If he has the box and papers with it, that adds some to the value.
For less than 98%, the value drops rapidly; with an average (say, 70%) finish, the value is only $200-250.
Jim