How can one deal with a coworker with whom one had a previous forbidden relationship, while ensuring the acceptance of repentance and remorse for what happened?

The sinful relationship with the man must be ended, for it is a gateway to evil and temptation, and it could escalate to immorality. You must be firm and decisive with him, and it is not permissible to allow him to be alone with you. Threaten to expose him if he persists. Similarly, it is not permissible to continue working in a mixed-gender environment except out of necessity and while observing the Sharia regulations.