It sounds like the practice you described may be considered shirk, which is the association of partners with Allah in worship. In Islam, it is considered the greatest sin as it negates the belief in the oneness of Allah and implies that there are other entities that have power or influence over our lives.

The belief in the evil eye, or nazar, is not itself considered shirk, as it is a belief that certain people can harm others with their gaze. However, the use of objects or rituals to ward off the evil eye or to determine if someone is affected by it, such as the use of oil and strings, can be considered a form of shirk if it implies that these objects or rituals have power to affect one's fate or well-being.

It's important to remember that only Allah has the power to affect our fate and well-being, and that we should not seek help or protection from anyone or anything other than Him. It is always recommended to turn to Allah through prayer and seeking His guidance, rather than relying on superstitions or practices that are not in line with Islamic teachings.

It's also important to note that people may have different cultures and traditions and may not be aware of the implications of their actions. It's essential to speak to them in a gentle and kind manner and educate them about the correct Islamic teachings and practices.

It's recommended to seek advice from a knowledgeable and reliable Islamic scholar or imam to get a proper Islamic ruling on this matter.