U may talk to ur friend properly about the whole matter.sometimes misunderstanding may occur but it is our duty to overcome those. Talk directly to ur friend and ask him what is in ur mind,dont doubt anything. He and his girlfriend has their issues, u dont interfare them. Concentrate on ur study career.

Despite being best of friends for so many years, it is really painful to see that your friend hasn't understood you as much as he has  taken his girlfriend's version.  The girlfriend seems to be possessive or insecured, hence could have behaved with  immaturity (by creating some misunderstanding between you both). You can make a choice between talking to your friend directly and assertively and clear/bridge the gap or let go of the friend; you are an individual and you have a life of your own. Just this issue must not be a hurdle/barrier for your life journey. There are so many other interesting people, things,  opportunities that you need to consider and move ahead in life.

Because, you are simply a friend and she is his girl friend. When the role changes, priorities also changes.