What is the ruling on fondling and kissing between an unmarried man and a married woman (his colleague at work), and what is their expiation?

It is not permissible to embrace or kiss a non-mahram (definition) woman, whether she is married or not. Such an act constitutes a grave sin and a confirmed transgression. What you perceive as a means to extinguish your desires may be the beginning of a greater indecency. You must turn your heart away from thinking about her, and avoid mixing with and being in seclusion with non-mahram women. If you are capable of marriage and fear falling into indecency, then marriage is obligatory upon you.