When you say “sin” do you mean mortal or venial? Not all inappropriate behavior is sinful. It could be a serious fault, which can lead to sin. Not following the official instructions for mass I think is not considered sinful after Vat II. Before for a priest to abuse the rubrics it was considered a mortal sin, much like meat on Friday used to be.
If you are talking about doctrine, then, that most likely could be a mortal sin, only if the said doctrine has no confusion surrounding it. For example if a priest/bishop taught against Limbo for unbaptized babies before Vat II, it would have been considered sinful by most lay people but now it is considered debatable. If it is debatable now it was debatable then, but not widely known, so depending on the education of the average lay person, who thought teaching against Limbo before Vat. II was sinful, and a well instructed priest/bishop who knew it was debatable, there is room for error. So unfortunately when dealing with sin one has to be precise in the offense and, as has been said, not deal with generalities.
On the other hand,
I do think that most Bishops are negligent to the point of being sinful (sloth or vanity --worried what people will think) whether mortal or venial I don’t know. Most Bishops are lost in confusion though, there is so much confusion in the Church that many just can’t get a straight answer.
We can know them by their fruits. Has the Catholic Church gained or lost conversions? I would say in general the Church is imploding in developed counties.(Pollyanna-Catholics would disagree) Many call themselves Catholic but don’t believe what is necessary and really could careless. Most of these pseudo-Catholics deny the teaching on contraception (confused more by the Pope and Homosexual prostitutes

)
How many sermons have you heard in your parish/diocese about

urgatory, Mortal /Venial sin, need for monthly Confession, True Presence of Jesus in the Eucharist, Contraception, Abortion, Stem Cell research…etc
Answers to objections: by Protestants, the new Atheist movement, Jehovah Witnesses,…etc
About the internal problems in the Church:
Modernism, Americanism, Validity of Novus Ordo Mass…etc…
(Unfortunately priests are so badly instructed that one said I was sinning because I told him after mass I couldn’t hear him, because the microphone was off.

Other than that he is a pretty orthodox priest - eccentric but good)
The Pollyanna-Catholics want to pretend everything is ok,( which I assume your friend thinks) but we are closing Churches and most Catholic families can say a majority of their families don’t practice the Faith, and if they do they have reservations on certain essential teachings. This destruction must be traced back to some sin/s that the Church has just not corrected.