The movie is very confusing, but the few clues suggest the following. Lisa married young and was probably bored. She had the chance to experiment and did so with the silly Ralph, a man whose life is purely on fantasy. She hid the affair from her husband for several years who was completely devoted to her, but when struck by cancer, she decided to leave him clues so that he could discover the affair. This is probably where your mind has to be a little twisted to follow her logic for doing this. In leaving naked photo evidence of her affair and clues to help her husband track down her lover the nut case, she could only expect that he could become enraged and hate her. Given how much they are supposed to love each other, one can only think that she was concerned that he might not be able to get over her death and so proceeded to make sure that he would hate her and in the process, get over her.

Peter worked out her plan in the end and that just made him forgive her and love her regardless - hence the 'thank god for Ralph' at the party and the comment to his daughter at the station 'she didn't know us at all did she'.