Plex Pass not allowing another account

I have a lifetime Plex Pass membership and my girlfriend has an account under me. I thought I had up to 15 people to share the membership with but it isn’t showing on her account and wants $4.99 to be able to cast to the tv from her iPhone. Am I misunderstanding how it works?

What do you mean by „she has an account under you“?

You need to add your girlfriend to your Plex Home for her to share your Plex Pass features:

https://support.plex.tv/articles/204234323-creating-a-plex-home/

So she is already on my account, i.e. she can view my NAS. Do I need to delete her and add her as a managed user?

She can see my the Plex server on my NAS.

No.
It appears you’ve added her as a „friend“. You’ll need to add her to your Plex Home instead. There should be an option to do that when hovering over her record in the friends list.