Let me start off by saying UGH... 
my friend of about a year has been dating this guy off and on for the past 6 months. When they aren't currently an item, he stalks her. I am not freely using that word either - he really stalks her. For example, he shows up at her work, calls her about 400 times a day, shows up at MY house looking for her, sends letters to her parents, emails all her friends, blah blah blah. It got to the point a few weeks ago that she was seriously thinking of filing a restraining order against him. I was all for that - especially when he showed up at my house.....Any who - i would stay up late talking to her giving her advice, helping her through this "tough time". Unbeknownst to me, she was still talking to this guy and leading him on! The real kicker is that they are back to dating again! When she finally came clean (after hiding their relationship for about a week from me) I was speechless. I hated her and felt sorry for her at the same time. She knew i was angry so she said "you said you would be my friend no matter what". To be honest, I don't think i can. This is serious. I really can't be some one's friend that lies, is dishonest, keeps things from me, and is a week person all around. I know that sounds harsh, but i would have liked to think more of her - and i don't. I haven't spoken to her in over a week and i feel like a huge weight has been lifted off my shoulders. I really do miss her, but i don't miss the drama that came with the friendship. If she would get rid of this stalker guy, i might consider being her friend again (not likely though) but as long as she is with him, i cant even stand to talk to her.
Has anyone else had a friend like this? Or been in situation like this? I feel so terribly bad about it, but i think what i am doing is the right thing....
My Best Friends Ex - Current - Ex - Current - wait, Ex - now current boyfriend
Wow! Thank you so much for such an incredible response! You have basically read the back of my mind and put it into words for me! Thank you so much! (i even have a tear in my eye....*sniff
*sniff). And to answer your question, no - we aren't in high-school, even though i think in her mind she still is. We are both 28. I am married and have been for a while and she was married
but divorced the guy because he was....well....the same as the current stalker boyfriend! He would grovel at her feet when they argued and she would make him PROVE that he loved her by doing
stupid thing. Come to think of it, she did mention to me that she enjoys the high of new relationships. Maybe she will always be a new relationship kinda gal - and when she starts to get too
comfortable and "boring" in a relationship she moves on. I have known her for many years, but only started being her friend a little under a year ago and i have never known her to be single.
But, she has always dated like 3 or 4 guys at the same time. Must be stressful. I couldn't keep them all straight! Hmmm...well - thanks again for your response - my eyes have certainly been
opened and i am looking at the situation at another angle and feel really good about it! 
I have had a friend like yours. It's constant drama. It's all 'fun' and 'juicy' when you're in high school (when drama is all time high), but when you're 30 like I am, NOT really fun anymore.
But I don't get mad when someone turn around and pull that cr@p on me...I've learned my lesson from high school. I've learned to not heavily invest emotion in my support for any of my good friends. I stay detached emotionally when I listen or give suggestion, so that way, I don't feel 'cheated' when s/he turns around and changes his/her mind completely about the jerk(s). And 90% of the time, they never listened to your advice!
What I usually do when someone pulls a 'BEAUTIFUL' turnaround like your friend: STAY AWAY from them for as long as you need to recuperate from those negative energy s/he's sending you
If you guys are still friends, the easiest way for you to 'neutralize' the negativity without too much involvement (while keeping in contact so that you get the 'latest' juicy gossip):
You don't pick up their phone calls (often)...well, she's probably excessively complaining to you (and mark my word: SHE WILL AGAIN, when their relationship starts to unravel) a number of time a day.
Next time: Just pick up ONCE a day. Or wait until the next day to call her back. Tell her you're a little busy too. If she wants to talk face-to-face, only make time for her as less as you can.
This is very effective way to give a subtle brush-off to a friend like yours while getting the update
So you're not completely abandoning her, but you're not completely involved so you don't feel this mad.
As for your friend's situation, here's what I see:
You don't like the 'stalker' bf mainly because of what SHE TOLD YOU.
You just found out that she's been leading him on anyway. The two of them are 'playing a game,' and she loves the drama if not, why would she be back together with him? Why would you want a guy who creeps you out (w/ stalking)? Why? Because she ENCOURAGED him to 'stalk' her, and he's the poor sap who knows no better but to grovel at her feet begging her to love him. Since she's leading him on, most likely she's actually implying that the only way for him to prove his love to her is by doing these excessive action (the 'stalking'--I hardly call it stalking because she WAS still talking and keeping in communication with him and then AGREED to date him again).
You should feel sorry for him (instead of getting mad, unless he has threatened your livelihood, in which case you need to file a restraining order against him), why in the world, a guy be so dumb, immature, insecure and obsessive of loving your friend..and your friend is just plain mean, but that's just my opinion. She does enjoy this drama and chaos. Tsk tsk. Not good. I hope you guys are still in high school because this is the type of behavior I'd see in high school.
Don't get involved anymore in any of their business. I have a 'bad' feeling that it's going to get worse between those two.
Good luck to you.
*Unconditional love is a beautiful thing, just be sure to give it AFTER your conditions have been met.*