Lily has two guides in House of Mirth – one for society and money, and the other for morals. Gerty is her moral guide, and Lily knows that she's the only person who can steer her through her time of greatest need – the moral crisis she experiences at the hands of Gus Trenor. Notice that Lily chooses to go to Gerty's place, not to try and find Mrs. Fisher, after the scandal goes down. In Book II, as Lily slides further into the abyss of poverty, Gerty is the one who feels she has a "moral claim" over our protagonist, encouraging her to maintain her moral standing even in desperate times. It's fitting that Gerty is present at Lily's death, to witness Lily's final rebuke of moral temptation (if you choose to interpret the ending that way).
Mrs. Fisher is Lily's social, financial, and even pragmatic guide. She plays a greater part in Book II than she does in Book I, starting with her decision to "leave the Brys" in Lily's hands and continuing with her various efforts to help Lily out after her disinheritance. Mrs. Fisher helps Lily into the less elite social circles. She even gets her a job at Madame Regina's when the social stuff fails. Notice that while Gerty is trying to help Lily be independent, Mrs. Fisher is encouraging her to marry – either Rosedale or George Dorset, both options which would require blackmail on Lily's part. It's fitting that Lily's social/financial guide and her moral guide are necessarily in conflict; more of that money/morality antithesis we talk so much about in "Character Analysis."