Love's sick game

Consider this scenario :

A guy, Y, has a best friend called X. They've been friends for quite some times and they are really close. Both are really smart people and while Y has the more nerdy look, although he's not, X has the more sporty cool look.
Now, Y likes this girl, S, and has a big crush on her. He tried to get close to her and know her bit by bit. They've become sort of friends and since X is always with Y, he also became friends with S.
One day, after months and months approaching her, he hit one her. She said no. He waited some time and he hit on her again. She said no, again. I'm not sure how many times he hit on her, but it got 'till the point that she got quite pissed.
The girl told all this to X and finally confessed to X that she actually likes X. Talking about a twist.
And suddenly they are a couple.
X didn't say anything to Y about this although Y found out anyway. Now, X and Y are not so charming buddies anymore.

The questions here, what should X do when the S confessed to him ? Is anyone to blame here ? Or is this just love playing a sickening game with these people ?