At first, Elena does plan to tell Lila what happened with the middle-aged Sarratore, then she changes her mind and decides not to. Her reasons for keeping silent, such as disgust with Sarratore and "revulsion" against herself, would, therefore, become the basis of the imagined dialogue with Lila.
Elena's part of the dialogue would be based on what she records happening in the book: Sarratore coming to her as she slept, kissing her, touching her, and arousing her, then telling her he loves her. Elena would describe how she kept her body still, and try to convey to Lila her conflicting rush of emotions, pleasure at the physical sensations from Sarratore and disgust that she "let this happen." Much of what she says could be paraphrased from the novel. She would tell Lila bluntly and truthfully what happened because the two are so close. There is no use pretending, as Lila would know if Lenu were lying.
Lila's response is trickier, as we don't have a solid guide. We know enough about her to imagine she would be disgusted with Sarratore's behavior. She would be on Elena's side at this point, and might ask her why she didn't fight him, kick him, and scream at him to go away, as Lila would surely have done with an unwanted suitor. Lila would very likely, in this dialogue, feel protective of her friend and offer to intervene and tell Sarratore off. Elena might then have to warn the volatile Lila not to tell anyone what happened and swear her to secrecy.