Why do you need to tell them its so bad? If they do everything else right, they're not going to go to hell for having bad taste in acessories are they?
Do you remember the story about the guy with the plank in his eye trying to get the speck out of someone else's eye? Maybe you should look for the plank first, then tell them about the speck ...unless you don't have a plank, or a speck. I think everyone has a speck at least, and if you can't see it, there's probably a plank in the way!!!
This is not the same thing. We are called to proclaim the truth sometimes in matters of faith. Wearing the evil eye is not so much a sin as trusting in non-Orthodox things for our salvation, is as such an issue of faith, and it would be uncharitable not to point it out. They are not fashion accessories but pagan symbols that reject trusting in Christ alone for our salvation.
If the person is aware of the Orthodox teaching and doesn't care, then I agree it would be better to leave them alone and pray for them and ourselves.