To me the red flag is "be sure she can trust you". Trust you to do what?
It doesn't sound like a good basis for a relationship, but I think it'll self destruct if you don't meet her. You need to take care of yourself in that situation and like above posters said, it's ok to set some rules yourself, like she doesn't get to dictate your relationship with him. Maybe my idea of a poly relationship is different from some other peoples' but I think everybody should be equal and independent and no one owns anyone else.