What should be done to a girlfriend of a lover who believes she is the legitimate lover of my male lover?

My male lover got this girl as his girlfriend nine months ago to hide our relationship apprehensive that people will talk about us when they find out we are having a relationship. He left the country only three months into their relationship to work abroad. I followed him and currently staying and living with him but he has not yet terminated his relationship with this girl whom he still entertains thru text messaging and phone calls which the girl occasionally makes. since he believes there is still a threat that people may find out about our relationship though I have disclosed the status of our relationship with close personal friends which my lover does not know. This however does not cause any problem. I am pissed off with this girl since this girl is desperately seeking my lover's attention believing she is the legitimate one... though she does not know the existence of my relationship with her bf ( my lover). They have only been together for three months. I have told my lover to start distancing himself from the girl and to tell her that long -distance relationships do not work so that it would be easier for her but until now, i do not know what the status of their relationship is. I gave an ultimatum to my lover yet at the same time i want this girl to stop pestering us. i had emotionally invested a lot with my male lover and the same with him for me so this is the dilemma. What should I do in order for this girl to finally silence her and stop pursuing my male lover?