How can I find out of a friend at work betrayed my trust?

I confided in someone at work ( a work friend, who works on another project) about a person who seats very close to me and talks non-stop on the phone and it is very distracting .. and that I might have to talk to my boss, because even the sound of her voice irritates me now. The next day is became very quite and starts working on something, which made me very happy of course..Since I didn’t complain to my boss, I am wondering if: A. Someone else complained; B. She realized herself that she talks too much on the phone; or C. My work friend said something to her, to keep her out of trouble. If the answer is C..Is it possible that my friend told her.. I think it would be kind of like a betrayal..and it means I cannot trust him to share my problems with any more.. is there any way to find out?