Keep in mind that Nick's set of values that places him above most of these characters is exactly the juxtaposition that Fitzgerald was trying to show. Nick is a character with moral roots built strong by life in the midwest (just like Fitzgerald); he is a better person than these people who have spent their time in the east (much like the people Fitzgerald was acquainted with after leaving Minnesota). It's no accident that Nick is morally strong while Tom, Daisy, and Gatsby appear to be quite morally corrupt.
I don't think Nick is immoral...perhaps confused and a bit star-struck, but not immoral. In Chapter One, Nick reflects on his time with Gatsby, saying, "what foul dust floated in the wake of his dreams that temporarily closed out my interest in the abortive and short-winded elations of men." Although Nick may temporarily suffer from a lapse in judgment, I do think he has a strong moral compass to guide him.
Even when he is enchanted by Daisy, the effect is only temporary: "For a moment the last sunshine fell with romantic affection upon her glowing face; her voice compelled me forward breathlessly as I listened--then the glow faded, each light deserting her like a lingering regret like children leaving a pleasant street at dark."
In the end, Nick reclaims his center. Of Tom he says, "I couldn't forgive him or like him but I saw that what he had done was, to him, entirely justified. It was all very careless and confused. They were careless people, Tom and Daisy -- they smashed things up and creatures and then retreated back to their money or their vast carelessness or whatever it was that kept them together, and let other people clean up the mess they had made...
Without a sense of morality, Nick would not have been able to reach any of these conclusions or see Gatsby and his crowd in a revealing light.
Nick is the moral center of the novel, and the term immoral is hard to connect with Nick. If you go quite literally, it could be seen that Nick spending the day with Tom and his mistress, as well as being his confidante, is immoral. Daisy is a relative of his, and it seems his compliance is in some way an acceptance of Tom's adultery.
However, it can be argued just as easily that he was in an uncomfortable situation, and just tried to keep the peace.