I also don't agree with the blanket rule that "one must never date a friend's ex". I mean, what if your friend dated a guy for 5 days in high school, and you're now all in your 30s, does that mean you're banned from dating him.
I think there are several factors to be taken into consideration, namely:
- the duration of the relationship;
- the seriousness of the relationship;
- who broke up with whom;
- is your friend truly over the relationship? Eg, are they now dating or married to someone else?
But I do agree that no matter what, it's important to be respectful of your friends, and considerate of their feelings.