This mistress did not wait for her boyfriend to cheat on her. The fact that he had hit on another woman made her uncomfortable even though he tried to pass it on as a joke and nothing serious. She had caught him and she knew if she had not, he would have gone further. So she punished him by spitting on him to make him realize she would not condone that.
Goddess Andova wanted to send a message to this guy and so she chose a method that would do that perfectly. She did not want to beat him up as it was not the time to use pain. She needed something to humiliate him but not make him feel pain. So she spat on him and she also made him swallow her saliva. That was enough and it did the trick.
This mistress laid a trap for her slave and when she caught him, she punished and tortured him with spit. She did not bother using pain. She knew that the degradation he would get from her saliva would be enough to make him think twice. Once he realized what was being done to him, he was so disgusted, humiliated and degraded that he promised himself he would never steal again.