First of, so sorry to hear this.

If she has left you once for another man, she will leave you in future. I do not see her as your future wife in any way. If you are heartbroken twice, it will be difficult for you to live life.

Nevertheless, it's your life; and choice is yours too. 

 I would not accept X in any circumstances because for her materialistic life seems more important than love, emotions and feelings. Now when Z ditched her and she's again looking for the love of Y but where was she when Y begged her to come back into her life. Even though i know that every girls are looking for a successful person in their life but why do they don't thnk this before even starting any relationship. I know its sound harsh and i am sure there will be few comments after mine against me but for me those X type if girl are a cheap a$$ biatchhhh...

Y should accept X, date a month and dump her and say he was ignored.

@hannspree: sorry bro..I know it looks like a mathematical equation and yes it is a f@cked up one!!
@all: Y is taking ur suggestion and he thanks you all. Any other different ideas here?

Grenade : This is common issue in US which you already mentioned and i have seen few cases as such too once the girl dumped the boy just for sake of some dumbass with PR forgetting the moments & relationship,memories she shared with boy the girl doesn't deserve the another chance but give yourself some time i know if you love someone it's hard to let them go just like that but before making any moves let the girl realize her mistakes and see how she comes out don't commit anything in rush .

Just stick as good friend without showing her anykind of emotions to her for few months if boy thinks she is good enough to be trusted they can start fresh relationship putting the past behind . I know several people who ended up making wrong decisions just give it a thought i am sure boys so such mistakes too specially these days where Int. students are more worried about their future rather then their studies .

 Y should accept X
  and
  dump her before she finds next Z to avoid next embarrasment

I suggest Y to stay away from X, she doesn't deserve his love. 
love is not something like in-n-out burger which you can eat when hungry and trash when full. 

-DrKanchho

I like nepalinetworks suggestion... That way it'll be way lot for u easier to take the next step. So, stick to her, not as a boyfriend but as a very close friend

 Don't know about X or Y or Z but i ended up taking Risperdal.

साला, म अझै सोच्छु म नै किन भनेर ?

You know, as far as i know, 

मैले आज सम्म कसैको केहि बिगारेको छैन.

FML.

x did wrong, bt if y is so depressed n is sure will be happy with x, and if y thinks his luv is big enuff to forgive x for the betrayal thn y shud accpt x back ..cz if x n y succeed this will just be forgotten in time

The concept of  "Once did and Will do it again" is completely wrong. Just don't rush, give both of yourself some time, do not see each other for al least 6 months, be a good friend, don't be too emotional. Within this period, if both of u realize, what mistake u guys had made( how  "Z'" was able to break the bank), monitor each other activities, write daily that helps to express your feeling comfortably, avoid predator friends, be positive, always think of a best moments that u guys have spent together, the most important thing is feel the necessity of each others love for each other. Blend this recipes with completely positive attitudes and see the outcome. The best input will always have a fruitful outcome. And, remember there is no guarantee that the next one in line will be better than u already have/had. people make mistakes, nobody is perfect.

Keep on rockin' 

 Y should be in his own way and should tell x not to bother him again as he has moved on.If possible use 'Jab We Met'.Burn all her gifts,photos etc.Y will feel lot better.She has already learnt her lesson by getting betrayed by Z.There is no point in Y betraying X again as X is human being.Better tell her clearly that x is in no mood to reconcile with her and try to find someone else, may be w

 Y should talk to X... make her fall in love with him and then find "W" (a PR girl) and dump Y. And then yell: "What goes around comes around!"

 hhehhehehe.... XYZW.... 

observe her again closely and analyze her behaviors for at least a year as a good fren. if she has truly changed then i don't think u should deny ur love... people makes mistake... and if u forgive her this time then it would be appreciated life long.  keep dating wid others as welll. if u find that it was just an another lie then simply move on.... don't get too emotional during this period...

@Rakchhas: I read ur thread couple of weeks ago. Are u and Y on the same boat? How are u doing these days?

@all: thank you for your inputs. Y has finally figured out that x had a "bad romance" with z. Y says there are not any chances under any circumstances that y will go back to x again and marry. That brings the story to an end.