Mr. Nightingale is a very, very small-scale foil to Tom. We say that because Tom and Mr. Blifil are clearly the real pairing in this novel: Tom is good, and Mr. Blifil is evil. Still, while Mr. Nightingale doesn't make mistakes on the same scale that Mr. Blifil does, his treatment of women does provide an interesting contrast to Tom's.
Mr. Nightingale is a good man overall, but he has developed slick manners from living in London. Like many of the other city folk in this book (such as Lady Bellaston and her circle), Mr. Nightingale treats seduction and love like a game to be won. He refuses to take seriously the feelings of the women he's sleeping with, and he brags about them all over town. Tom tries to get him to stop, but it's a total habit by now.
We can compare Mr. Nightingale's behavior with Tom's respectful treatment of Molly (at least, up until he realizes that she is not that attached to him) and Lady Bellaston. Tom doesn't love either of these women, but he worries about their reputations and he tries to do right by them. Tom may not be the most faithful of guys—he tells Mr. Nightingale that he has been "guilty with women" (14.4.2)—but he does his best not to hurt anybody.
Mr. Nightingale's shallow attitude towards women almost ruins the life of Nancy Miller. He seduces her and she gets pregnant. Mr. Nightingale tries to treat Nancy the same way he might treat any of his other lovers—by dodging her and trying to avoid any responsibility for the consequences of their relationship. But he forgets that Nancy isn't some hardened city woman: she truly loves Mr. Nightingale, and she tries to kill herself several times after he tries to abandon her.
Thanks to Tom's steadying influence, Mr. Nightingale returns to Mrs. Miller's house to marry Nancy properly. The two of them settle down in a house near Tom's in Somerset, along with Mrs. Miller and Nancy's little sister, Betsy.
We can't help but notice that, by this late stage in the novel, Tom is starting to grow into his role as a Squire-Allworthy-style influence for the good. His moralizing speeches to Mr. Nightingale about how to treat women properly almost match Squire Allworthy's well-meaning monologues to Tom from earlier in the novel. Obviously, Tom's moral transformation over the course of the book is coming to a positive end.