1) Give 6 Nectars to Megaera
2) Keep talking to her n’ fight the furies whenever possible, until the 2 key scenes with Meg occur in Zagreus’s bedroom (shown in video)
3) Give 1 Ambrosia to the babe to unlock her Battie companion
https://store.playstation.com/#!/tid=CUSA00572_00
source
For anyone struggling to get Meg to go to zags room, I needed to buy the bat cage for the lounge before she would even after 130 runs.