Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
If the thickness of the eyebrow is so abnormal that it makes her look ugly or causes her to feel embarrassed, then she may remove the additional hair.
Answering your question, Dr. Muhammad Salama, PhD, Islamic Studies in English, Associate Professor at the Faculty of Islamic Sciences al-Madinah International University (Mediu)), states:
The Messenger of Allah (peace and blessings be upon him) is reported to have cursed an-namisah, a woman who plucks other’s eyebrows, and al-mutanammisah, a woman who has her eyebrows plucked.
According to the majority of scholars, it is not permissible to remove the hair of eyebrows whether by plucking or shaving. According to the Hanbalites, however, it is plucking only that is impermissible while shaving is not impermissible. This means that plucking is unanimously prohibited. Some scholars even included it among the enormous sins. However, shaping eyebrows by shaving or any other means is controversial as mentioned above.
Accordingly, if the eyebrow has a normal shape that does not make her look ugly or cause a woman to feel embarrassed, then she has to avoid reshaping it. If the thickness of the eyebrow is so abnormal that it makes her look ugly or causes her to feel embarrassed, then she may remove the additional hair, according to the Hanbalite point of view.
Allah Almighty knows best.