Is it ever okay to date a friend's ex?

I'll try to cut a very long story short here: One of my friends was dating this guy for 7/8 months. Eventually he broke up with her but it got very messy - she lied a lot about things he'd said/done, just generally kicked up a massive fuss, and even pulled the pregnancy card on him (spoiler: she wasn't even remotely pregnant). I get that being dumped sucks and everything, but this was excessive. Anyway, I became friends with this guy while all that stuff was going on, and became even better friends with him after it had all died down. We hang out quite a bit and enjoy each other's company. Then he asked me out on a date a couple of weeks ago. Of course, I had to turn him down on principle because he'd previously dated this friend of mine, despite any feelings I might have for him and the fact that they split up almost 6 months ago now. This got me thinking. Is it ever okay to date a friend's ex-boy/girlfriend? Would it ever be worth the trouble it would undoubtedly cause? Is there any situation where doing that would be acceptable? Has anyone here ever done it?

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