Is the orange that my brother picked from a tree in the garden belonging to his workplace, and shared with his colleague, considered unlawful (haram) because it is not his property and is considered common property for all employees, especially since the division was not fair?

If the garden's fruit is equally shared among all employees, then none of them may take exclusive possession of the fruit or take more than their share. The jurisprudential rule states that equality in possession necessitates equality in entitlement. The Messenger of Allah (peace and blessings be upon him) forbade "qiran" (eating two dates at once) unless with the permission of one's companions. Al-Nawawi mentioned that qiran is forbidden if the food is shared, unless with their consent. Therefore, what the questioner stated regarding her brother's lack of right to exclusively possess the garden's fruit and her refusal to partake of it is correct.