If... You are becoming increadibly attracted to your ex's best friend, and you are not sure whether you should go out with him because he makes your hearth throb with strong sexual and aggressive emotions, or not go out with him because you feel it is "ethically" or religioslly, etc INCORECT... You need to venture outside the little donut shaped reality of your current life and just stick your toe in the water of the larger world and really just experience life alittle more. Ok... SO.. yes he may definately be a catch and may be a REAL man (i really stress on that) and you may be attracted to him, BUT... while your chasing his tale for him, there very quite possibly will be a man just around the corner that your not really noticing. You may not be there when that opportunity comes along?....Now this is my point of view and nothing to tie you down over... but it is really just the sort of thing you wanted to hear... some good advice --> yes it is allittle paradoxical as its not the answer you were dying to hear... but it is a consideration.
However...i am going to take your (go out /be involved) as a relationship type inteaction and not simply friendship... and in that case, it is a decision that you are going to have to take... to find the answer you really NEED.
P.S. If your on a break with your bf (I really dont see the point - to me it says I dont feel anything when im aroud you!!!!) and your digging his best friend... Why stay unhappy and sit at home for hours on end ripping your hair out and tying it in knots, if your bound to this guy your not sure about?