That must have come as a huge shock, Polarbear and, as your father died 40 years ago, you may have lovely memories of him that have taken a radical shift.
It was not this friend's place to tell you.
As your mother knew, she probably thought it best to turn a blind eye and keep the family together and it must have been so difficult for her, but at least he didn't abandon you and you still will have memories of a lovely father.
If they lied, they did so to protect you. Perhaps you now need to rationalise all this and come to terms with it, as your mother did so stoically, before you decide whether you want to find your half-sibling.
Best wishes