What's the code of conduct here??

At what point is it okay and appropriate to date someone who is married or has been married? Let's say a guy is married but separated from his wife. Can you date him? Or are you supposed to wait until the paper states they are legally divorced? Oh and just to clarify, I talking about being with someone who is no longer committed to their spouse and both parties are on the same page of not being committed and have broken up.