Hey, I know exactly how you feel, I'm in a similar situation with my boyfriend. First, what you need to do is plan things for you and her to do together that take up a lot of time. Make plans to do something in advance so that way if he happens to ask her to hang out she will already have plans with you. The second thing is you need to be upfront with her, I understand how it is when they get angry because my boyfriend is the same way but you need to tell her that she is your girlfriend and you want to spend time with her as well. And lastly, you need to ask yourself if you trust her, if you do then you should believe what she says about him being just a friend. I know how hard this situation is and it does really suck but It helps when you know others are in the same situation. I wouldn't get yourself too worked up about this, if you and your girlfriend are happy with one another then it'll be okay. Everything will work out, good luck!