I want to go out for breakfast with my best friends boyfriend
Why would you want to go eat with him? Can't you all go together?
My best friend is not going to be there.
I think that you should skip out on this. You should not want to go behind her back to see her boyfriend.
What do you care?
Only because it may seem a little suspicious to have you go out with him behind her back... she might get the wrong idea. How about instead waiting for all three of you to be available so you truly mean no harm?