You weren't to know the truth, and your friend's dad baited you into it. What did he expect you to say in response to his actions? If he'd asked you to guess if the photo was really his twin sister, or his face photoshopped onto a woman's body, your reply would have probably been a little more tactful, and thought out. As it was, you were only told it was a photoshop job, and you reacted to the idea of seeing what your friend's dad would look like dressed as a woman. I'm not saying the way you reacted was right, but if I were in your place, my first thought wouldn't have been "maybe it's really his twin sister", so you shouldn't be faulted for taking it all at face value, just by what you said, and how you said it.

How do you have different gender identical twins? Identical twins come from the fertilized egg duplicating resulting in two fertilized eggs with the same chromosomes, meaning they’d be same gender. Fraternal twins are two different eggs being fertilized and having differing chromosomes and potentially different genders