I had this card come up a lot when I was going through a depression. To me The High Priestess represents like others have said someone who has cut themselves off from reality. It is a very intuitive person who is very connected to the spiritual world. But it represented a time where I isolated myself from others. I had a hard time coping with the demands of real life. So I spent a lot of time thinking about things in my head. Which can be a strength (in moderation). I think the High Priestess gives a warning to find a balance between the physical world versus the spiritual one. To make sure that you are focused on the reality of your situation vs. letting your emotions get the better of you.
Also with the High Priestess there is great intuitive abilities, but sometimes the intuition can be misguided. I became so reliant on my intuition and created this own reality in my head which was so far from the truth. I created so much pain and confusion in my mind, when nothing in the physical world had actually taken place. It was all in the mind.
I was so passive when it came to life, I felt like I just had to take all these things that were happening to me, but wasn't actually taking any action to solve my problems. Again felt trapped by my own thoughts- they in a way held me back from taking the steps I needed to sooner.
Hopefully it's not too confusing, this is just my experience with THP as mental health
The thing is with the High Priestess is there is a double standard. THP expects to know the deepest darkest secrets, but is very guarded when it comes to sharing anything about them. You could have known this person for years, but at times they still feel like a stranger. So I can imagine how your friend must feel if this is her partner. …the 2 of cups I think points that she must continue to nurture and love the partner but more importantly the partner represented by the High Priestess needs to learn to love himself. Whether he finds that through counselling, a new hobby (whatever it may be) …he needs to take the time to heal himself from within. Because he won't feel truly loved until he experiences self-love.
I remember when THP kept coming up- I wanted to be loved so much…but I had to ask myself the question: how can i expect someone else to love me if I don't even love myself?