That's Harry talking, not one of the fae who knows. Check anytime the fae say it, as my point was about their grammar/syntax. Harry can still lie. And of course our ignorant little narrator doesn't have the same insight the sidhe have, he's often just wrong too.
However Mother Summer answers;
"That's why we use that word rather than a name, Sir Knight. Yes."
Splitting hairs aren't you? Or trying to at any rate, whether or not Harry is saying it,makes no difference in this case. In general I'd not argue with what you are saying, except in this case context is important.. As chapter 33 of Cold Days opens Mother Summer is taking Harry by the hand to the Outer Gates on page 225. First thing out of the gate Mother Summer asks Harry what he thinks will happen if he disobeys Mab's command? We know what her command was, to kill Maeve. Why? Because she is infected with Nemesis beyond curing. Harry says she'd get pissed if she doesn't and he could end up like Lloyd Slate. The Mother asks, "if she isn't?" I presume Mother Summer means if Maeve isn't infected and he kills Mab instead because she is the one infected.
Then Harry says that Maeve will end up with Mab's mantel. Mother Summer asks him if that would go well for him... Harry doesn't think so and mentions someone who likes to pull the wings off of flies. "Well, crap," were Harry's words, and Mother Summer agrees.. Then the conversation proceeds about "the adversary," Remember at this point Harry isn't sure that it isn't Mab who is infected, after all he did see her on ice back in Proven Guilty along with obviously infected Lea. This is one of the main topics of the whole book, Mab commanding Harry to kill Maeve because she is infected. Harry not sure what to do because he isn't sure it isn't Mab who is infected..
Now comes a bit of conversation that is a little choppy so can be confusing unless you follow it closely. Page 326
"Well, crap," I said.
"Quite so," said Mother Summer."And if you do heed Mab's command?'
Mother Summer just asked what Harry would think would happen if he killed Maeve as commanded?
Then the line gets a little confusing because Harry stops his sentence mid-thought to ask Mother Summer if he can say
the adversary safely."Maeve's mantle gets passed on to someone else," I said. "And if. . . the adversary? Can I say that safely?"
So if Maeve is infected, he kills her and someone else becomes Lady.. However if he listens to Maeve that it is Mab who is infected, but she isn't, and kills her.. Infected Maeve becomes Queen. However his thought is interrupted, "..." because he asked Mother Summer if he can use the term adversary safely?
She answers;
Mother Summer smiled. "That's why we use that word rather than a name Sir Knight."
Then in the next line, Harry has realized what would happen if an infected Maeve were to take over Mab's mantel.. He isn't talking about a general adversary in this sentence, and the context of the two page conversation between him and Mother Summer is not about a general adversary or enemy, it is about him being commanded by Mab to kill Maeve, and Harry's indecision because the thought crossed his mind that it could be Mab who is infected.. Both decisions have consequences one a little more serious than the other..
So Harry thinks it through.. What happens if he decides that Mab is the infected one and kills her and Maeve takes over the mantle of Queen? In other words, Maeve/the adversary, gets the Queen's mantle.
"If the adversary has taken Mab," I said, "then it gets to choose an agent to take the Winter Lady's mantle. Two thirds of the Winter Court will be under it's influence."
It is pretty clear I think that in this case, "the adversary" is Nemesis and not some other general foe. Why Jim chose not to capitalize it? You will have to ask him, or the Beta readers, they are the ones I think, that correct these kinds of errors if they catch them.