I have this friend, N, that has recently started believing things very out of the ordinary. He said that he's been getting into paganism and studying runes and candles. I told him that was very interesting, it sounded like a fun time and a good hobby to have. N let me know that it wasn't just a hobby, but that it had a function and purpose, a sort of witchcraft.
I come from an academic background; it doesn't seem healthy to me to actually believe and try to practice witchcraft, not because it might be real, but because of the mental health associations with it. So I told my concerns to N but he brushed them off, saying that people have the right to believe in what they want. I respect that, but I do not want N to fall into delusion. I let him know, as respectful as I could, that believing in witchcraft does not make it real and that it wasn't healthy to believe otherwise. He got really bothered when I told him he should talk to his therapist about this stuff. N has a history of mental health issues and was in a deeply abusive relationship for a few years which cause him a lot of grief. I was accused of being rude and trying to impose my beliefs. His last message to me was him asking me to stop and that anyone has a right to believe what they want.
I cherish N a lot as a friend, I do not wish them any harm. I respect that anyone has a right to believe what they want, but I really don't think this is healthy for my friend. Is there any way I can help him? Is the best way forward to just stop and let him be?
Choosing to genuinely believe in superstitions seems to be a common coping tactic for people who have experienced extreme mental distress. It is not your place to try and "heal" them, they are probably in the process of healing themselves. That is what therapists are for, to help them heal themselves. So it is great that your friend is in therapy, it looks like they are on top of things for now.
But this is not necessarily unhealthy though. It can be unhealthy for some people, but not everyone. It is really a case-by-case kind of thing, and it was a little presumptuous of you to say that, which is probably why they got upset with you. For some people, tempered belief in superstition might be a natural part of their healing process.
Of course, things can still go bad, but you can keep an eye out for any behavior that might be harmful toward themselves or others without being rude. If they do (not believe, but actually do) things you think might be harmful, speak up and address that specific behavior, not their beliefs.
And, being another person to hang out with outside of their new circle of witchcraft friends could help keep them grounded, as long as you have things to do that you both find fun together that are not related to witchcraft.
If I were in your place, I would be honest that I don't find witchcraft to be interesting or helpful for me personally, but but I feel happy for them that their spiritual journey seems to be helpful for them, and that their well-being is the most important thing for me.
Be honest and supportive, keep trying to have fun as friends together.