Hi.
She reminds me of Me, and it seems she might have AS.
I was told by my Ex that I only ever "opened up" just after I woke-up i.e. when I was very tired, but soon when wide awake I changed into my usual introverted self. I didn't notice it. Its got to do with I worrying about what the reactions will be to what I say. When half-asleep, I just spoke about whatever and didn't worry at all about emotional reactions, which are difficult for me to predict.
If you can't predict another person's emotions easily, conversations tend to involve emotionally neutral subjects such as e.g. the Weather, for fear of causing offence and evoking emotions that not well understood nor handled. I am thus always polite and formal. Discussing emotionally charged relationship issues - opening up - involves talking about annoyances, being honest, saying what you like or dislike about each other, talking about the future etc. Later on, I tried to open up, and I noticed the point when I changed from speaking about an emotionally neutral subject to talking about an emotional subject, it was like rebooting with a different OS. I'd pause and then start talking in a different mindset.
Also, if you're asked to open-up and to talk about something, it usually means its a serious conversation that will involve negative emotions. When negative emotions are involved, it causes me as more stress, you end-up hypervigilant against causing further accidental offence. You don't want to cause any more anger. Very awkward. So that can be another reason for not wanting to open-up.
But I think it is possible for her to open up. What to do...
I also open-up when I drink c. 2 glasses of wine, I think. As I said, I open-up when I'm very tired such as when waking up. And I have managed to open-up when specifically asked to do so, but its awkward. I also read that writing down ones feelings and discussing things on paper or even in an email, rather then speaking, can help some people.