The "No Dating Your Friend's Ex" Rule

Ye olde "golden" rule:
"Never date your friend's ex."
Come on, girls, seriously? LEAVE THAT SHIT IN MIDDLE SCHOOL. 
Honestly, I've always hated that rule.
Because that person is your friend's ex; a.k.a., they're not together; a.k.a., your friend literally has NO say in who they date (or you, for that matter).
If you won't get with a guy SIMPLY because he once dated a friend and/or because your friend would get pissy with you because you're dating an ex of her's, you either need to
A.) Grow up
B.) Get better friends. 
Because that is a very silly and childish reason for not getting with someone, and if your friend has a problem with it, then they obviously have some sort of attachment issues that they need to get over.
The only time this rule is okay is when that person was a toxic person and/or if they hurt your friend really bad.
So other than that, unless that guy (or gal) cheated on and/or was a straight-up horrible person to your friend, I say, GO FOR IT.
I didn't get too deep into the topic on my post because I didn't feel it was necessary, but some lovely ladies have, and they have made some very good points.
Of course, things are a bit different and are in a more grey-area if the relationship between the guy and your friend was very serious or even if they were married, versus, say, a high school relationship. And of course, the amount of time that they've been broken up plays a factor as well. I mean, it might not be the most respectful thing to swoop on in if they've only recently broken up, for example.
If you ever have any doubts about it, talk with your friend and see how they feel about this potential relationship. And yes, they may be a bit upset about it, but you need to remember that they're broken up, and they're broken up for a reason. They may not have been compatible, but maybe you and him potentially could be VERY compatible.
Obviously, this isn't a black-and-white issue, but at the end of the day, it's your happiness that matters. Do what's best for you.I