I would say no if its a friend that is worth holding on too. If you see the world in this one girl then yes but if its because she is pretty and she flirts with you and she might sleep with you then NO! Remember there are many girls in the world that are prettybut when you meet the right girl you will know, and if that means she just happens to be your friends ex then go for it.
Not so much an ice breaker. I already broke that rule with two different girls just cause of their hotness. And yet this third girl who I like more on a I would date long term and same vice versa.... We both didn't date each other because I said no at first. Shes okay but as i got to know her she became so pretty, more so than even the other girls.. You know? But then later I realized how great she is and she ended up with another guy.. And now I got with a girl. And the spark is still there.
We both barely see each other anymore cause both out bf/gf started seeing it too.