A reader writes:
If the parent of an adult knew his son was going to confession, and told the priest in advance, instructing the priest what kind of advice to give his adult son, and if the priest cooperated in giving that advice, does that violate the seal of confession in some way?
Given only the facts that you mention, the answer is that this would not violate the seal of confession.
For a violation of the seal to occur the priest must, in one way or another, disclose both the identity of the penitent and the sin that was confessed. It is not sufficient for a violation of the seal to simply say merely "John Smith came to me for confession" or merely "Someone confessed to me the sin of adultery." However, if the identity of the penitent can be linked with the sin confessed then there is a violation, even if the linkage is accomplished in a roundabout way (e.g., "'John Smith came to me for confession and someone--wink, wink, nudge, nudge--confessed the sin of adultery").
N.B.: While saying "John Smith came to me for confession" does not violate the seal, the disclosure of even this amount of information would give Rome the willies in most circumstances.
From the above it is clear that no violation of the seal occurs if a priest merely listens to advice regarding what he ought to say to a penitent. As long as he (directly or indirectly) discloses no information about the penitent that can be linked to what the penitent confesses then the seal is not violated.
This is a very dangerous situation, however, because a priest listening to such advice might even inadvertently disclose such information--e.g., by thoughtlessly saying, "Good idea; I'll mention that next time," or even too enthusiastically saying "Thanks!" or just nodding at the wrong moment or appearing too interested in the advice or even just listening to it for long enough that the mere fact of listening sends the signal that the advice is relevant to what he experiences in the confessional with the penitent. Any of these--if the advice concerned a sin that the penitent was likely to confess--would allow the identity of the penitent and the nature of the sins confessed to be linked, and thus some form of violation would occur.
The safest thing for a priest to do if offered such advice by a parent (or anybody else) would be to refuse to listen to it.
Failing that, the priest would need to set up clear and explicit hedges to block the inference that the parent's advice was relevant--e.g., "I have to inform you that I can in no way confirm anything that your child may confess to me. I will listen to what you have to say, and I will hear you out regardless of whether what you say is relevant to your child's confessions or not. Nothing about the fact that I listen to you or how I respond can be construed as an endorsement of the idea that you are saying anything at all that is relevant. In fact, I may deliberately assume facial expressions that would be misleading on this point, just to keep the matter confused."
That, however, is a very unsafe course, and it would be better for the priest to refuse to listen to the advice.
The above deals with situations in which the priest could only have learned about the sin confessed in the sacramental forum. If the same sin was known to be known to the priest from independent sources, the situation would be different.
For example (and to use an uncommonly clear example for purposes of illustration), suppose the priest was in a bookstore with the parent and the child. While there, the priest and the parent mutually witness the child take a book off the shelf, slip it under his shirt, and walk out of the store without paying. So both the parent and the priest unambiguously know about the sin without the sacrament being involved. If the parent then turns to the priest and says, "You should know that Johnny suffers from kleptomania. He's receiving psychological treatment for it and often feels very guilty afterwards about what he has done. If this comes up in confession then you ought to tell him to listen to what his therapist is telling him to help him get over the problem" then the priest wouldn't need to go through the elaborate rigamarole I described above. In this situation saying, "Okay; good idea" wouldn't betray anything that the priest had learned in confession. Indeed, the confession wouldn't have occurred yet.
Between these extremes is a spectrum. In drawing a line on the spectrum regarding what the priest can't do without violating the seal, the line is to be drawn at the priest doing something that discloses information about what the penitent has told him in past confessions. This can happen either directly (e.g., "Thanks! Johnny confesses that to me all the time!") or indirectly (e.g., implying that Johnny may well confess this in the future, when the only ground the priest may be thought to have for supposing this is likely to be past confessions). In evaluating where the line is to be drawn in practical terms, the priest must err on the side of caution and do all that he can to not reveal information about past confessions, either directly or indirectly. In practice, refusing to listen to parental advice is likely to be the best way to do that.
However, if he has listened to the advice then the fact that he acts on it does not violate the seal. The seal protects information from going out of the confessional, not into it. If a priest hears good advice, from whatever source, that may be useful to him in confession (whether it is reading a moral theology manual or listening to an overly-intrusive parent) then there is no violation of the seal if he acts on that advice.
Would the priest not at least be under obligation to inform the penitent that he had spoken to his parents?
There is no canonical obligation in this regard. A priest is not bound to disclose to the penitent the sources that inform whatever advice he gives in the sacramental forum. Just as he isn't canonically obligated to tell the penitent what moral theology manual his advice comes from, he also isn't obligated to say whether a particular person (parent or otherwise) gave him the advice that he chooses to pass on.
One might argue that there is a moral or prudential obligation to tell the penitent that a parent has been talking to him--lest this fact come out later and cause a family ruction--but the same argument could be made that he should not reveal this on moral/prudential grounds (i.e., a family ruction would be more likely if he told on the parent).