I think most people have heard of bro code or girl code, which states that "one shall not date a friend's ex," but is that always true? Is there a situation where dating a friend's ex would be totally cool?
Bad Idea: Your friends ex has a title on the end of their name, as in ex-wife/husband/mother of their child/father of their child.
Don't do it you fool! Even if they say they are cool, they are never going to be cool going over to pick up their kid, from the house they used to live in that you are now shacking up with their ex in, to see you what, also trying to mother or father their kid. This will end your friendship.
Good Idea: Your friend only dated their ex less than 6 months
Six months probably wasn't all that serious, but just make sure you don't just immediately try to hook up with the ex. Give your friends dead relationship a little air, and then only with their blessing, date away.
Bad Idea: When you ask about dating your friend's ex, they flip out but approve
If you ask if you can date your friend's ex, and their response is something like "f-ck it, I don't care, go ahead and date that a-hole if you want. I hope you get screwed like I did." Uhhhhh, don't do it. Your friend is clearly still pissed and or hung up about the ex and even if they don't want anything to do with them and never will, they aren't going to be supportive of your relationship if God forbid, s/he treats you 100 times better than they did to your friend or you make it last and get married, or you want to have a conversation about them, to your friend, or the same thing happens to you (you can expect an I told you so, rather than any support). Trust, your friend will be bitter and/or take it out on you.
Good Idea: You are not really like best friends
Everyone always talks about the girl/guy code as being something you don't break, but if you haven't really known your friend all that long and if they suddenly got pissy and said, we can't be friends anymore because of you dating their ex, and you're first thought would me more like, okay, whatever, you wouldn't be devastated at all, date away.
Bad Idea: You played therapist for months/years
When your friend was dealing with her boyfriend/girlfriend, if you were the go between, or the somewhat therapist to all your friends feelings, angst, hurt, and pain at what happened during their relationship throughout the months or years, unless that was like 20 years ago and they've since very much moved on, don't do it. Your friend WILL feel betrayed like you were just there waiting in the wings for the break up to happen, even potentially egging it on, getting insider info to try and screw him/her over to date the ex.
Good Idea: Your friend has definitely moved on
If your friend is happy now, has kids with and/or is married to someone else completely different, dating their ex shouldn't be a problem with their blessing of course. It's kind of hard for a friend to say don't date their ex, when they are in a totally different relationship now.
Bad Idea: They still talk
This is where you have to know just how jealous or messed up you would be if you were dating their ex, and he or she was still hanging out regularly with your friend, or texting them in the middle of the night, or still a little too close for your comfort. If their break up was really amicable and they remain really good friends, this may be a super bad idea. Your friend had first claim on them, and their ex nor your friend may not care about your feelings if there is even a hint of a spark still there.
Good Idea: They both admit their relationship was super casual
If your friend and the ex make it seem like it was just sex, or super casual and no feelings were really involved, dating your friends ex, again, with their blessing, could be good. Just make sure that you define what it is you want in that relationship at some point whether casual or serious, because you are not your friend.
Very Bad Idea: You were clearly the reason your friend broke up with his/her boyfriend/girlfriend
If you are literally the reason your friend calls their last relationship, an ex, now dating them is not going to make things better. It may not have been your fault at all...maybe the ex was openly flirting with you despite being with your friend or trying to get you to cheat with them, because they wanted you over your friend so badly. You will 100% end your friendship if you get involved with her/him now. If you weren't reciprocating any of his/her advances during your friends relationship with her/him or you told your friend about what their ex was trying to do with you---activating the break-up, and trying to solidify your friendship---then dating them will only confirm what your friend only suspected but could never prove about the two of you, that you were trying to cheat or were cheating behind their back.
Good Idea: Your friend is the reason s/he now has an ex
If you watched your friend take advantage of, screw up, screw over, ruin all hope, cheat on, and otherwise destroy their relationship with their ex, they seriously have no right to tell you, she or he is off limits. If your friend was the problem and the reason why the two of them broke up, that ex is fair game, especially if you are nothing like your friend. Everyone deserves happiness and if your friend has a problem with you finding happiness with their ex, then when they were dating, they should have treated them well in the first place. That's nobodies fault but their own. Date away.