Hi Dada,
He's your best friend, and she's your girlfriend. These are two people who you should be able to be really honest and open with. As such, the best thing you can do is talk to them
But when you do, remember that they are both free people.
It may be simply a natural maturing of their friendship, which can often lead to these types of behaviours, and if you let them know that it bothers you they may naturally change to ensure your happiness
However, before talking to them, why not take some time to prepare yourself for 'the worst'.
When two people fall in love, there is very little that can stand in the way of that, even friendships. If they are in love, then there's really nothing you can do about that So the question for you is, how can we make the best of it? By letting them be together, by letting go, and by removing them from your life, for a while, until you feel able to join with them as friends
I sincerely hope this isn't the case, but if it is, take it as an opportunity to grow, and deepen your ability to forgive, and move on
Your true strength is far beyond what you may expect
I hope it goes really well....
Love one another, but make not a bond of love:
Let it rather be a moving sea between the shores of your souls.
Kahlil Gibran