Answered by Shaykh Yusuf Weltch
Is it permissible for a woman who is losing her hair due to sickness to cover her head with a wig or a hair topper?
In the Name of Allah, the Most Merciful and Compassionate
It is permissible for a woman suffering from a condition that causes her to lose her hair to cover her head with a wig or a hair topper, as long as it is not made of human or pig hair.
Human hair is impermissible to use due to Allah Most Highs ennobling of the human being. This applies to any and every part of the human. Pig hair is prohibited as it is intrinsically impure. [Tahtawi/Shurunbulali, Hashiyat Maraqi al-Falah]
Additionally, the color of the wig or the like should be a normal hair color. This is an addition that I have heard from my teacher Sayyidi al-Habib Umar, and perhaps the reasoning is that a colored wig (i.e., not a natural hair color) resembles sinful people.
Hope this helps
Allah knows best
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani