Is what happened between a girl and her male colleague, namely the development of their feelings and friendship into love and a relationship, considered zinā (illicit sexual relations), especially since she has become fearful of God's punishment and feels remorse?

It is not permissible for a woman and a man who is not her mahram (definition) to have a friendship, as this falls under the category of illicit intimacy (mukhādanah) which Islam has forbidden. The desire for marriage, when love occurs, is a good thing, as the Prophet, peace and blessings be upon him, said: "There is nothing better for two who love each other than marriage." However, this desire is not a justification for the continuation of communication between two non-mahrams. Rather, this relationship must be severed, and the marriage should be initiated, while asking Allah for ease. And "there are three whom Allah is bound to help: the fighter in the cause of Allah, the mukatab who intends to pay off his debt, and the one who marries seeking chastity." Mere heartfelt emotions are not blameworthy as long as they do not lead to something forbidden. And one who has reached the age of taklīf (accountability) is not described as young.