This is sad but like you said you aren't dating exclusively and she has every right to hear anyone out that she wants to. Now if you two were committed to one another it would be a different story. You should either ask her to be exclusive with you or wait and see what happens. You have no boyfriend powers/rights in this situation even though you would like to be her bf. If you dont want her meeting this guy ask her to date you and only you and if she refuses then you might think about leaving.
One day the Moon said to me, "If he makes you cry, why dont you leave him?" I looked up at the Moon and said, "Moon would you ever leave your sky?"