You’ve actually asked several questions so let me take them one at a time. You are jealous of your friend talking to someone else because you are insecure. That’s not at all unusual in someone who is 13. You are at an age where you are figuring out who you are and how to fit in with other people.
None of your tactics with J will work because the problem isn’t J. The problem is in you. You need to work on gaining more self-confidence. The way to do that is to leave J alone and work on being the best person you know how to be. Take care of yourself. Do some volunteer work and other things that make you feel good about yourself. I’m not talking about giving people advice. People generally don’t like to get advice unless they ask for it. I’m talking about doing things to make the world a better place.
As for your friend who has visual hallucinations: She may be merely someone who has a very vivid imagination. But if she feels it is totally out of her control and it scares her, she really should see a professional therapist for an evaluation (not your friend at university and not you). If she does have an emerging mental health disorder, it is best to get treatment early. All you can do as her friend is make a caring suggestion that she should talk to a professional. She has to decide for herself if she wants to follow through.
I wish you well.