You could also invoke Dynamis and become a strong influence, so as to attract her to you.
Whenever I've had jealousy issues regarding a girlfriend/potential GF and someone else, Ive strengthened myself and focused on my personal pursuits. My love interest is then naturally drawn to me... or other love interests are drawn to me, who end up being a better match for me.
Just thinking outside the box. It can't hurt
What I'm saying is, sometimes the harder I try, the more I push someone away. My interest in life becomes them, and they find me boring. If my interests in life are wide and mysterious, they are more likely to become interested in me.
I've been in your shoes...it's very frustrating. I've found the problem doesn't lie in the situation itself. If your lady friend wants to talk to this person, it's her perogative. Give her a reason to want to talk to you, instead. If she doesn't see that reason, it's her loss!!
It's late and I'm rambling