A good scientific theory must explain the data, but it must also be vulnerable to the data. The validity of the theory must be put at risk when we make observations or do experiments. There must be things that, apart from the theory in question being true, we would reasonably expect to observe, but which are prohibited by the theory.
However you said here, in response to your correspondent's complaint that ID "is compatible with *any* pattern" of evidence, "that makes it the best theory then, because it best fits most of the evidence."
This is wildly perverse. It's saying that a theory can be consistent with any possible observation -- that is be invulnerable to the data and therefore untestable, and still be good theory. This is most emphatically not the case.
When organized matter is found to behave in accord with predicatable laws, then it is reasonable to attribute this to intelligent design. What is intelligent design but taking matter and then organizing it to behave according to predictable laws?
Actually this is the opposite of ID. Propents of ID claim to infer "intelligent design" in precisely those case that (they assert) CANNOT be accounted for by the natural behavior in the form of predictable laws!
What's more you're taking a presupposition shared BY all scientific theories (the uniformity of natural law) and saying it's a prediction OF a particular theory.
Who says? Why carry the attribute of "vulnerability" into the definition of theory? A theory by defininition is simply a way of explaining data. Read the definition again, and tell me how you wring "vulnerability" out of it.
So? That makes the theory even more applicable, since it encompasses the practice of science itself.