they could of easly aranged an albi way in advance she could of come down and spent the night there to give him an albi.
acording to her he told her he was planing a long time before the murder so they would of had plenty of time to set up an albi together.
Then he would have to have admitted HE did it and he would have to have selected the exact day in advance as opposed to picking a day of opportunity like he did. You are asusming she would have helped him though ther eis no guarantee she would have.
Worse yet whatever they come up with would be DIFFERENT from their usual schedule. Deviating from their usual schedule exactly when the murders occur is supicious. Her visiting out of the blue to provide an alibi would be extremely transparent.