Jealousy!? Help?

My boyfriend has a girl best friend whom I do not care for. There have been some iffy incidents that I did not like very much. I have opened up to him about it multiple times. Nothing has changed.

Yesterday he took her to fast food at 2am when I politely asked him not to out of respect of our relationship, and he took her anyways. Is this okay? I honestly would rather them not be friends. I feel betrayed and Everytime I open up about something concerning her, the only excuse he has is "she's my friend, you should trust me" but I'm not quiet there yet. Help?

Thank you