Many people assume that you could never date a friend's ex while others take the "finder's keeper's" rule, which is right? Neither is, you really have to look at your friendship and go from there. If you are looking for a very clear answer that will not require that you sneak around or confront your friend about such issues, than the answer is absolutely do not date your friend's ex.
Of course, there are some circumstances in which you may be able to date a friend's ex without breaking any serious dating or friendship rules. You may be able to date your friend's ex if:
Your best friend has since moved on and is in a serious relationship with someone else
They have been broken up longer than they were together
You have talked to your best friend about you possibly dating their ex and they said they would be fine with it
If your friend has said in a meaningful way that they don't mind that you date their ex, and more importantly, that you believe them
It can be difficult when you like someone a lot and you want to get closer to them to look at what your friend wants, but you need to be a friend first and foremost. The problem that most people have is separating their crush from their duty as a friend.
You need to preserve the friendship first and foremost, and then consider a budding relationship. If you aren't sure that your friend would really be alright with your dating their ex, it would be better to hold off. You don't want to lose a friend because you want to date someone that may or may not turn into a long term or serious relationship. There are a lot of fish in the sea, and some of the fish are off limits and others may just be off limits for awhile.
If you like your friend's ex the best thing you can do is talk to them about it. Be honest and let them know that you have had some very minor interest in them and that you wanted to know how they felt about it.
If you tell them that you won't date them, stay true to your word. You really need to mean what you say, so think through your responses before you speak so as not to promise someone something that you will not follow through on.
