It looks like you're assuming several things. That X told Y malicously and deliberately to hurt either you or Y.

That Y is gullable and stupid, or that X is stupid and suicidal.

What is it that makes Y beleive X more readily than you? HAve you perhaps given Y cause for doubt already and X just filled in a few missing gaps unwittingly?

If you know enough about X's activities to blackmail X (which is what B1 is, an intent to blackmail), then why would X deliberatley tell Y stuff to hurt you?

Could it be that X has been hurt or let down by you in the past or made to chose between you and a hypothetical Z? MAybe you pushed them too hard. They are, after all only human and just as suseptible as you.

If X has been a freind for so long (10+), then you must have some idea what they are like, have they done this sort of thing deliberatley before?

Likewise, if you've been freinds for so long, maybe X knows you too well and doesn't want to see freind Y get hurt knowing that Y is less able to take it than you.

Whatever, you have to talk to X and Y and the hypothetical Z if there is one. If you threaten X with telling all, are you sure there isn't something else they know that they might use against you?

Or maybe, your consceince is telling you something you don't want to listen to.

DW