What would you do if you saw your boyfriend with your best friend?

Depends... If I seen them just talking I would trust them enough to not be doing anything but still keep my guard up. I would not show any sign that it bothers me because that just gives my boyfriend and best friend a reason to be mad at me. If I seen them kissing or hugging in a way i know there is something going on, I would definetely move on from my boyfriend and consider my best friend no longer my best friend.