Talk to your friend. It's that simple.
For example. My closest friend used to live right next to me and he had started a (sexual but not official) relationship with a clerk at a corner store around the block. I knew of her and always saw them together. One day before going out we stop by the store to buy god knows what and he talks to her and goes back in my car, I still have to buy all the stuff so I'm waiting in line and she start flirting with me. I don't respond to it, but I'm nice. After buying the stuff- I was prolly in your position. lol She was a very good looking girl.

So I tell him. and go figure, he doesn't care.
Of course, it all depends on how close you are but since you said best friend- he shouldn't care and should definately NOT put anyone in between you two.

friendship is one of the most important things about life.