Should I have asked out my best friend? A simple case of boy and girl.?

If she's in a relationship still I would say to just let it be for now. But yes, you did fall for her. Or at least it sounds like you did. Anyway if things go south with this guy then be right there when he does and once a lil time pes and she's over him, try to make a move. Since you've been friends for so long then if she doesn't want to it wont change anything. But if she says yes you'll most likely have an amazing relationship.