It's possible Deb didn't expect Lisa to sleep that long. Perhaps she was totally expecting Lisa to be up at 11 or 12 or even earlier. Maybe Lisa was early to bed, early to rise? Maybe she sleeps from 6:30/7:00 until 4 or 5ish. I know plenty of babies who sleep through the night at that age. If she was put down at 6:40, bathed and fed and comfy, there would be no need to physically check on her. Listening for her, yes. I don't know too many parents that get up every hour to check on their 10 month old, even if they have a cold with a cough. Baby isn't feeling well, let her sleep.
Being aware and alert is another story, but that was not even negligence in my opinion, it was foolishness. Not to mention, she expected Jeremy home earlier... But it also did not cause Lisa to be missing imo.