Black thread on wrist to ward off evil eye

My wife had wrapped my son wrist with black thread, according to her that it will keep him away from evil eye, I quoted her one of PBUH hadith from isalmqa but she said it is on amulets, Tawiz and not on a piece of thread that is there with intention of 4 quls so he doesn't get eye evil. The problem is that every time I tell her something which her mother or anyone else in family is doing, she wanna prove me wrong or his brother try to be a scholar. How can I encounter this?