You posted that you'd known x a long time, you don't say how long you've known y. You also don't say how you found out or if what y is saying has upset her is the truth or has enough of the truth that you recognise it or if it sounds like a complete fabrication.
If it is a complete fabrication, the question may then be who made it up? (who is more likely to lie to you?)
or is it a case of what was intended not necessarily being the same as what was heard or understood.
If on the other hand you recognise what was said - is friend x the only source, friend y could have got it from? If there is no doubt that friend x has betrayed a confidence - the question is then was it intentional, & why?