How can I deal with a colleague who abuses another colleague using obscene language and inappropriate gestures, and threatens him, to stop his actions?

You have thought well about speaking to someone who harms others, and you must restrain him from his injustice, for the Messenger of Allah, peace and blessings be upon him, said: "Help your brother, whether he is an oppressor or oppressed." And the way to help him when he is an oppressor is by restraining him or preventing him from injustice. The conversation should be with wisdom, good admonition, and gentleness, reminding him of the Hereafter, while observing the appropriate time and place.