Its a hard lesson to learn - basically you cannot trust anyone with secrets. I found out that the hard way. I would suggest that you do nothing to X, but remember what X did and don't put yourself in that position again. I believe in Karma. KB is right, no one is all good or all bad, you though would have thought that after 10+ years, you would have an idea of X's loyalty. Perhaps X has gone mad, or deliberately wants you to break her/his confidence cos she/he is too chicken to do it.
Anyway, I hope things get better for you and Y, my very best wishes to both of you
your friend