This girl was a snitch and when mistress Nicole and mistress Kira found out about it, they had to punish the girl and teach her never to snitch again. She had messed up and she had to pay for her mistakes. That is why the mistresses chose to use their boots to trample her and foot gag her until she felt not just pain, but also humiliation. She got the message.
This mistress was not happy or pleased that her boyfriend had been doing the wrong things intentionally. So she chose to do something to get back at him and to teach him a lesson that she was sure he would not forget in a hurry. The mistress enjoyed dominating and humiliating him in a way he had never been before. And that is when he realized that she could be lethal too.