Dating your best friends ex is not a good idea. It crosses the line of friendship and it could even ruin your friendship.
It could get very awkward when it comes to dating your best friends ex. You might still have feelings for that person and seeing them together will just upset you. You don't want to see them with someone else let alone with someone you are close too.
You might feel that you can't control your feelings but it is still wrong. Think about if it done to you. You might be okay with it but when it actually happens you might not be. So before you make any rash decisions and risk ruining a life long friendship think of how you would feel if it was done to you.
There are plenty of guys in the world. Yes you can't help who you fall for but you could help what you do about it. Just because you have feelings for someone does not mean that that person is right for you. Dating your best friends ex is off limits. It is easier said then done. Think about your friends feelings and how you would feel if your best friend was going out with your ex. It's not a good feeling so don't do it.