if they are a real friend to both of you this person should still stay your friend after the break up and explain this to your ex. if your friend drops you just because of your break up then they were never a true friend.
it depends on how strong your relationship is. if you both are not fully secure about each other i wouldn't do it. but if its something to bring some extra spice and both of you trust each other then it shouldn't mess it up. you both want it right?