Suppose someone approaches a priest or spiritual director to ask for advice about a moral dilemma. Then, suppose what that trusted adviser recommends is sinful. If the person, in good faith, acts on the advice, is he still culpable for the sin?
I imagine at least it could not commit mortal sin in this situation, since the person did not act in “full knowledge,” even if it was grave matter.
If one ask a priest for advice and the advice is sinful or wrong one act in good faith. If we can not trust a priest who can we trust? I do doubt a priest would act against the official doctrine but if one does not sin. On the other hand, The RCC have never said we should not use our own brain, if it feels wrong, don’t do it, if you do, I don’t think it is a sin. It is always about circumstances, a thing a priest or confessor must think about, still, I can’t think a priest would give a sinful advice.
It will depend. Limiting myself to only what is presented above, I would say that culpability would be reduced if not eliminated. It will depend on a number of factors that are specific to the given situation and the individuals involved.
I base my position on:
a) The fact that this person has some authority over you and you regularly meet and depend on them for their guidance.
b) That you are acting honestly and in good faith and,
c) That the recommendation has not been made lightly - but has been discussed at some length.
In such matters, I would tend to give the priest a bit more latitude than a non-priest spiritual director. The priest has the authority to forgive sin where a SD does not. So - if you discuss a given situation and the various options with a priest, the advice given carries with it a certain level of “forgiveness” if there is sin involved. A lay SD who cannot forgive sin needs to be much more careful in such a situation.
Another factor in regards to culpability would be just how much "good faith’ is present on the part of the one seeking advice. Has he presented the dilemma accurately? Is he truly looking at all the possibilities? Has he tried (consciously or not) to steer the conversation and solution to what he wants?
The Catechism states that “feigned ignorance” or hardening of the heart tend to increase rather than decrease the seriousness of a sin (and culpability). So it is vital that one be sure that they are indeed acting in good faith.
As you see - there are a number of caveats and conditions that come into play in such a situation.
It might help you to remember Lk. 12:48, “But he that knew not and did commit things worthy of stripes, shall beaten with fewer stripes.”
God will determine the culpability of sinfulness according to one’s full knowledge of their wrongdoing, or lack thereof.