From a reader:
Except when he translates for or overhears another person’s confession, is a layman ever bound–like the priest–by the seal of Confession, such that he cannot mention either what he confessed or what he was told? I was not able to determine this with certainty, but obviously it would be important to know with certainty if the answer is yes.
Except in the cases you mention, that is, when he may serve as a translator or when he overhears the confession of another, a layperson is not bound to keep secret what he or she says or hears from the priest in the confessional. If a translator or "over-hearer" were to reveal the contents of a confession, they could under the Church’s law be punished with a suitable penalty. The "Seal" which applies to the confessor does not apply in the same way to the penitent, even if the penitent is a priest. In the case of a cleric overhearing a confession and then revealing the contents, I suppose a penalty might include dismissal from the clerical state.
But in general a person can reveal he contents of his own confession and what the priest says.
That said, it is probably better for the penitent not to speak too much about what occurs in the confessional under normal circumstances. The less said about concrete instances of the sacrament of penance the better.
There are some exception to this, of course. Off the top of my head I can think of both positive reasons and negative reasons. For example, if a priest gave a particularly good piece of advice, perhaps that might be shared if you were not also going to reveal your own sins as a result. Otherwise, if a priest were to do something outrageously stupid or attempt a crime or the like, the penitent could and should address himself to the priest’s superior about what happened. The difficulty is, of course, that the priest remains bound by the seal and it could wind up being a matter of the word of penitent "A" against Fr. "B".
So, the long and the short is that a penitent in general can speak of his own confession and the advice and penance received, but in normal circumstances it is better to leave it for the most part in silence.