Welcome to the forum. I find what you're doing with your research really fascinating.
I'd ask you how close are you to that person? I think it would be a little odd if you intervene in a person's activities out of nowhere. Otherwise, if you have some degree of connection with that person, it would be easy to have a conversation about it.
Also, just leaving a letter seems a little "sneaky". How about striking a conversation, developing some rapport and maybe depending on what you'd get from that person, decide if/how to say something about it.
I'd suggest to avoid coming across as some person that comes out of nowhere to warn somebody. That could actually backfire and spark more interest instead of the probable desired effect you want to cause.
Normally I'd say "mind your own business" or something along those lines. Since you seem to have done some research, maybe sharing that with the person, without trying to influence their decisions, might be helpful.
At the end of the day, people are free to choose what to believe, study and follow.