I'm having trouble with this scenario, probably because we don't have enough information yet.
PH and co-worker are going to carpool to the bar to meet other co-workers and listen to music, which starts at 10:30 PM, IIRC. PH leaves her car at the co-worker's house, so it's obvious how she plans to get home from the bar.
PH checks in with her family regularly all day long, possibly as part of her on-going recovery. She tells her father about her plans for after-work entertainment.
No 'check-in' calls to her family after work or from the bar?
The friend sees her leave, or doesn't see her leave, but the friend eventually feels comfortable going home without PH. Does she leave without knowing if PH is coming back or how PH plans to get home? Does she just basically take off without knowing whether or not PH is still there? I don't see any indication that PH checked in with her friend before she disappeared. We also haven't heard a peep from the other co-workers - when they arrived, did they see PH, when did they leave? And I'm mindful that we don't know how much this friend may have had to drink and how clear her thoughts and recollections might be.