I know of what you speak. There is no mistake, I can assure you. This particular priest absolutely, without a doubt in either my mind or my friends mind, broke the confessional seal. I pray for him often.
You do not "absolutely without a doubt" know unless you confront the priest, preferably with your friend as witness, to ask him to verify that he did or did not reveal a particular sin that you told to him
within the sacrament only (without not also mentioning it outside of the sacrament during direction, a private meeting, etc.). He will ask what you are talking about. You will repeat what your belief is, based on this third party's version of what he said. He will then ask the friend to recount the conversation between him and her, such as, 'You, Father, discussed TxTrad's sin that she confessed just a few days ago,' and precisely as she confessed it. You [brought it up out of the blue]" -- difficult to believe, by the way; priests have better things to do -- or "[you, Father, responded to a question of mine about sin in general, and you chose to insert into our conversation a particular sin a penitent, my friend, had just revealed to you in the holy sacrament of Confession.]"
Etc. There are obviously other scenarios, but none of them are believable to me, because they all assume far too much, to wit....
The confessor has a morbid interest in your personal sins, outside of the confessional, and is willing to risk his position and his standing with the Church, just to satisfy his morbid interest.
In turn, that would mean that he has an intense, spiteful, or vengeful dislike of you for some personal reason such as some "bad blood" between you and him. But it would have to be pretty extreme for him to risk so much just to indulge a capital sin of Anger, Envy, etc. There are a lot of jerky priests within the Church, although they are a minority, but very few of the worst jerks would jeopardize their position unless they were already on the way out, by their own choice or someone else's. And he knows darn well he will have to answer to a very justly angry Second Person of the Trinity for indulging a vice at the expense of his own soul and in violation of a sacred trust. I don't care what trad or N.O. seminary he graduated from; he knows it.
Alternately, the priest thinks so highly of your friend that he figured he could "score some points" with her by revealing a sacredly privileged secret. If so, either she is someone very important in the parish -- so much so that he found it useful to ingratiate himself to her by going so far as to risk his soul and his position, or he has a personal stake in his relationship with her. Men are more prone to sudden heart attacks than women are, and often those first heart attacks are final heart attacks. He would have to think very highly of your friend to make such a foolish error, particularly because he would also know/assume that she would repeat it to you -- assuming he knows you two are friends.
Less far-fetched (slightly, but only slightly more believable) would be if he repeated your sin to another priest. That would be less risky to him because he knows that other priests know what canon law binds them to, as well as what it bound your confessor to. Therefore, unlike your friend, any priest hearing your sin outside of a confessional would not be so foolish as to repeat the sin that was revealed. Your friend seriously compounded the problem by either misunderstanding a general comment to equal a specific revelation, or she participated in the same hypothetical evil he committed by then detracting against him. Detraction is a mortal sin. If she were so convinced that what the priest said was an actual unlocking of a secret confession, she should never have told you. She should have brought the situation to her own confessor to ask for advice on how to handle it. If I were she -- your friend -- and I was absolutely convinced that this was a deliberate breaking of the Seal on the priest's part, I would not have gone to you, but I might have gone to my bishop if a (obviously different) confessor did not satisfy my sense of Catholic morality.
At the very least, she participated in backbiting, which is a specific sin, and in this case a very serious level of backbiting. Possibly she participated in calumny or detraction. And if she misunderstood what he said, she threw you into a great deal of anguish about this, anguish of soul, anguish of heart and mind -- threatening not only your relationship with this priest, but possibly your confidence in approaching the sacrament, period.