“Let not the magic arts of that worthless Sanford lead you, like an ignis fatuus from the path of rectitude and virtue!” Lucy Freeman writes Eliza. She tells Eliza that a rake can never be reformed; however, even if Eliza could reform Sanford, she should still reject him because his debauchery and lack of “virtue” are offenses “no degree of repentance can wholly efface.”
An “ignis fatuus” is a reference to an old folktale about a phantom light that leads travelers astray, and Lucy likewise suggests that Sanford is leading Eliza away from virtuousness. Lucy claims that Sanford can never repent for his lack of virtue, which is to say, indirectly, that Eliza won’t be able to either.
Lucy questions how Eliza, a woman “used to serenity and order in family” who is also so “refined,” could so easily “relinquish” these qualities for a “whirlpool of frivolity” and “licentious wit.” Lucy reminds Eliza that her happiness in this world and the next relies on her behavior. “Reverse the scene,” Lucy warns.
Again, Lucy’s lecture is incredibly degrading and critical. She implies that Eliza is too good for her current behavior and that her morality is suffering. It is ridiculous to assume that Eliza is immoral simply because she is friendly with Sanford, but Lucy implies that Eliza is headed for eternal damnation.