What is the ruling on female colleagues boycotting their un-hijab-wearing colleague at work, which has negatively affected her? And what is the optimal way to deal with her?

Abandonment (hijr) is a means to reform, not an end in itself. If it does not achieve a benefit, but rather leads to harm (such as apostasy or an increase in evil), then it is not legislated. Instead, what is legislated is conciliation and advice. Ibn Taymiyyah said: "Abandonment varies depending on those who are abandoning. If the benefit outweighs the harm, such that it leads to a weakening of evil, then it is legislated. However, if it increases evil, then it is not legislated; rather, conciliation becomes more beneficial." The Prophet, peace and blessings be upon him, reconciled with some people and abandoned others. It appears from the question that the boycott was mistaken and did not achieve a benefit, and the preferable path is to follow the way of gentleness in da'wah.