How can a woman regain her chastity and purity after having made a mistake with her colleague, especially since she was known for her religiosity and good morals?

It is understood from the question that the book was not written between the two engaged parties. Therefore, what happened was due to the intermingling of men and women, which is a fitna (temptation/strife) that diminishes a woman's modesty and leads to prohibitions and the corruption of households. It is obligatory to repent to Allah and to distance oneself from situations of doubt and fitna. Allah is Forgiving and Merciful; He accepts repentance. The questioner must regret the sin, abandon it, resolve not to return to it, avoid seclusion with a non-mahram man, and wear hijab in front of him.