Is it ok to date your friends ex ?

Is it ever ok to date someone your friends dated? If you meet a man/women out in the streets whether it's at the store, bar, or club and you realize after the initial conversation and name exchange that he/she dated your friend what should you do? Is the deciding factor how long ago they dated? What if they only had sex once or twice or not at all? Should you have to call your friend and get the ok from him/ her? If your friend says they have an issue with it are you obligated not to date the person? There are many questions to be asked on this topic. How would you decide? What would you do?