hit the screen yet and you are already on to the next part of the story?! I'm not about to hold someone accountable for what they did when they were mentally unstable. She was crazy then aka not responsible for her actions.
Ava and Nina's friendship formed onscreen so it's not like it's out of nowhere. Ava is an intelligent woman and I truly believe they have never discussed it because Ava knows Nina was cuckoo for cocoa puffs back then. Nina also is the only one who believed Ava when "ghost" Nikolas was haunting her and she helped her solve that mystery and get out of Ferncliff.