Most Helpful Girl
What exactly do you mean by Asian child? As in Asian features? I only ask as down-syndrome used to be called mongolism(a horrible term) simply because a downs baby will have narrow slanting eyes, a short flat-bridged nose, a flattened skull. The baby can also have jaundice causing a change of color to the skin.
Of course this may not be the case at all but simply that she cheated. If that's the case then you are perfectly entitled to walk away, actually run away. You owe her nothing and at least you know now, not in 20 years time after raising a child as your own. I will never understand how anyone could do that to someone they supposedly love.
You need to talk to her, even if its just to get closure of some sort.