What it sounds like, whether you want to hear it or not, is that your friend has found a way to attract attention to herself and to get you paying a lot of attention to her. You know that Slenderman isn't real. You know that she may have imagined what she saw. Your Tarot reading even said that what she claims is imaginary. Yet she persists because every time she does you get all flustered and start acting like she is telling the truth.
My first suggestion would be that the next time she tells you this you look at her and tell her, "I think you are making this all up." and then drop the subject. If you don't react to her claims I suspect they will stop.
On the other hand, if you are truly worried you could either make her a protective talisman to carry, or you could create a shield spell to protect her. I doubt these are needed, but if it will make you feel better then go right ahead and do them, But if you do, you are feeding into her game and she will likely come up with something new the next time she feels your attention isn't centered on her.
Oh, and please remember not to type in capital letters. It's against site rules.