I just assumed he told her he was coming to ride over with her. It's also usual for someone to watch the baby while the mom gets ready. With the pecan thing, I wondered if she had sent him to get pecans while she showered and that's why he was in Walmart with the baby.
It makes sense that she knew he was coming over, she hadn't put on makeup or fixed her hair and needed to make that casserole. He comes over, she sends him out to get pecans with the baby and she gets ready. He goes to walmart to get diapers and formula because he's going to kill her and will need baby supplies. He goes back to the house, kills her and then takes the baby home for dinner.
Personally, I think the casserole is going to play a part in all of this. I'm sure she made it, and it was either in the fridge or he brought it.