Summoned to Duty....

So recently, I got a message from a good friend saying something to the extent of "I broke up with (longtime boyfriend).  If I possibly consider getting back together with him, please come knock some sense into me".

Now, I've been summoned to the duty of "break-up buddy" many many MANY times in my life.  I have always given the job 100% dedication.  I have never faltered on insisting that said friend should NOT get back together or casually hook up with said ex (and I have always believed so 100% in my heart).  I have never failed to remind the friend not only of small pet peeves but also of large scale character and relationship flaws that most likely have not and will not change.   I have been tough as nails. 

However, all too often, said friend has ended up at least backsliding at one point.  Said friend has become all hopeful that said ex is the most wonderful person in the world (believe me, NONE of my friends' exes came even close).   In fact, often, said friend LIES to me about her backslide.

I want to be a good breakup buddy.  And I have surely tried my hardest.  Have I failed?  (I feel like I have, each and every time.)   What can be done to keep the friend from going back to and hooking up with the jerk?  Has anyone out there been a truly successful break-up buddy?  Can one be a successful break-up buddy?  How?  

I am currently on duty and I know I will be summoned to duty again soon.  I want to do a good job.  Help.