Black magic does exist and when it happened to our nabi (swt) Allah Ta'lah sent verses down for us.
Suratun Nas (114) and Suratul Falag (113) should be recited. Read the translation in english to understand what it means. Its interesting.
To prevent evil eye, read above surah's, pray five times a day, read Quran daily and inshallah you will be fine.
So what do you say to someone that claims to do all of the above, and more, and still is troubled?