Opinion on dating a friends ex?

What is your opinion on kissing or dating a friends ex (boyfriend or girlfriend)

I have recently had a friend who kissed one of her best friends ex boyfriends 2 weeks after they broke up. The two friends are no longer speaking.

Is it acceptable because they have broken up or is this always a bad thing?
Update: Also, do people agree that the two friends will never really talk again, or should they forgive each other and put their friendship above the guy?
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