Is it considered a sin not to offer advice to a knowledgeable and learned person, and does that entail participation in his sin?

Advising people by enjoining good and forbidding evil is a religious requirement, and one who abandons advice and forbidding evil is sinful. Allah the Almighty commanded the believers to enjoin good and forbid evil, and He cursed those among the Children of Israel who disbelieved for not forbidding one another from evil. The Prophet, peace and blessings be upon him, said: "Religion is advice (Nasiha)," and he urged changing evil with one's hand, tongue, or heart. If people see evil and do not denounce it, Allah is likely to encompass them all with punishment. The one who advises must choose the best and most beneficial methods of advice.