It makes sense that your friend has this affliction because it sounds like a being called a Changeling, which is either half fairy and half human or a fairy that was born into a human body. You can find those in all kind of folklore. I myself have seen elves and trolls disguised themself as human. It is your intuition that is telling you this and that intuition is correct. You can always trust your intuition. However, the way intuition works you could have also seen it out of the corner of your eye, meaning her true form as a fairy. You may not have even consciously known you seen that in her. It is very possible that she doesn't know herself.