Changing someone's world view that's backed up by experiences isn't easy, so it's likely not the best target. Even if you could destroying the fantasy world in which an emotionally fragile person withdraws is no good idea.

I would rather focus on making her feel safe and helping her to be emotionally stable.

But what do you do when she tells you about "unusual" experiences? I would recommend to listen and ask her questions like: "How does it make you feel that the spirit stands guard?", "How does it make you feel to tell me this story?" and "Is there something new in this experience that you didn't experience in the past?"

That can help her order her thoughts. Not focusing on the content but how she feels about the content is likely to help you to listen in a nonjudgemental way because you can be genuinely care about her emotional experience.

If she doesn't do any sports I would encourage her to do some physical activity to get more in touch with her body.

More seriously, my girlfriend has told me stories about seeing and interacting with "spirits", although she's asked me not to repeat any of them

Then repeating them in this way in a public forum is quite a breach of trust.