Look the problem with women, and you will find this out with a little more time buddy is they think emotionaly not rationaly.
Play her emotions on the subject, which might sound weird but I guarantee it will work. Literally tell how you feel and and make it sound sad. If she really cares about you she will understand and will stop doing whatever it is that bothers you. Weither it be hangin with another guy or whatever.
Obviously if she cares about you more than whoever this other dude is, she will stop hangin with him.
Hope it helps bud.
P>S. My advice, stay single.