On the Aristotelian account of ethics, what is "good" for an agent depends on that agent's essence/nature/form as well as that form's characteristic natural ends. For example, it is good for a dog to reproduce, eat and drink, and have fully working senses, the reason being that reproduction, internal equilibrium, and various types of perception are some of the natural ends (in Greek, 'teloi') of a dog. When it comes to us humans the general outline remains the same, though we will have some differences insofar as humans are essentially different from dogs, and hence characterized by different natural ends.
Now, it seems plausible to think that there are necessary ethical truths. For instance, it is always wrong for people to torture little children just for the sake of it. But could the above Aristotelian account contradict this? It seems that, given human evolution, the conditions whereby natural selection brought about the existence of humans might have brought about evil counterparts, call them "anti-humans." All of their natural ends would be inverse to ours. For instance, while it is good and fulfilling of our ends to protect others, for the anti-humans it might be good and fulfilling of their ends to commit murder. Similarly, it might be good for them to rape, torture children, commit genocide, etc. Hence, what is good or bad is simply relative to whether you are a human or anti-human. Call this idea "Aristotelian Relativism."
There are a couple of ways an Aristotelian could get out of this relativism. First off, there is the theistic route. Given God's existence and the idea that goodness is grounded in his nature, this would provide an objective basis for the good which holds across all possible worlds. This prevents the instantiation of any species whose ends include raping and murdering their young, or something of that sort, thus making the anti-human scenario strictly speaking impossible.
There are also a couple of ways for a non-theist Aristotelian to respond. The first is to bite the bullet and say that such things which appear horrendous to us (and indeed are horrendous for us) would not be horrendous for some other kind of thing. It's not a reply we might like, but that's exactly what we should expect given that it is contrary to our nature to do these sorts of actions. When we see rape, cannibalism, and slaughter in the lower animals, though we are likely to be disturbed, we nevertheless admit that it is natural to those animals to bring about and flourish from these types of suffering.
A second and maybe more satisfying way to respond is to say that such a scenario is not metaphysically possible for a species of rational animals. Remember, though they may disturb us, we already acknowledge that cannibalism, slaughter, and forced sex can be fulfilling for some lower animals, as there are examples of this found in the natural world. But with rational animals it is different. Aristotle understood the essence of man not to consist in our purely biological properties, but rather in our rationality; hence, "rational animals". It is from this that he derived the virtues and goods that we already acknowledge. But that means that for anything which can be classified as a rational animal, it will be wrong for it to do the things such as murder, rape, and torture. Hence, the idea of an "anti-human" turns out to be contradictory. If anti-humans are rational animals, and Aristotle's derivation of the natural ends of rational animals is correct, then it's metaphysically impossible for murder, rape, and torture to be good for such creatures. Anti-humans, then, are metaphysically impossible, and the things we understand to be good for persons are necessarily good after all.