Why is my boyfriend still talking to her?! Pleasee answer..as much advice as possible!!! Guys too!?

[Elizabeth Andrews]

So a while ago, my boyfriend was facetiming a girl and she had a tank top on with her boobs hanging out..and she screenshot it posted it so thats how i know! And i got mad and talked to him about it. He said he was sorry and he wont do it again. Well i have been stalking his twitter page and he's all like...i love you too bestfriend<3 and all this stuff and he knows i dont like her. But it's a different story when its me. I haven't even tried to text or facetime other guys bc i know he would be upset...and hes always questioning what im talking about with my guy friends...
Its not really fair! And they text all the time. I dont care if she is his best friend...i dont like her. Or what i hear about her. He knows that...so why is he continuing to snapchat, text, and factime her?!?!???? >
Guys perspective?
Ill take anything at this point...bc im about done.
Everyone i appreciate your adive..but just know that she didnt flash or show her boobs, she was just purposely making them look big and pushed up in her little tank top. But to top it alll off he was shirtless...which is sketchy..