“I am speaking now of the highest duty we owe our friends, the noblest, the most sacred -- that of keeping their own nobleness, goodness, pure and incorrupt.”

I understand the sentiment, not sure on its implementation.  To care about the well being and honor of one's friends is integral to friendship, IMO.

Owing a duty to our friends is a religious tenet — not obligatory or lawful. Religion is: “ real piety in practice, consisting in the performance of all known duties to ... our fellow men.” (Bouviers Law Dictionary) It is pure religion that keeps one's self unspotted from the world. Such honorable and virtuous venture assists others in their nobility, goodness, purity and incorruptibility.