Rule 2.11 of the Model Code of Judicial Conduct identifies situations in which judges must disqualify themselves in proceedings because their impartiality might reasonably be questioned including cases implicating some familial and personal relationships, but it is silent with respect to obligations imposed by other relationships. This opinion identifies three categories of relationships between judges and lawyers or parties to assist judges in evaluating ethical obligations those relationships may create under Rule 2.11: (1) acquaintanceships; (2) friendships; and (3) close personal relationships.
In short, judges need not disqualify themselves if a lawyer or party is an acquaintance, nor must they disclose acquaintanceships to the other lawyers or parties. Whether judges must disqualify themselves when a party or lawyer is a friend or shares a close personal relationship with the judge or should instead take the lesser step of disclosing the friendship or close personal relationship to the other lawyers and parties, depends on the circumstances. Judges’ disqualification in any of these situations may be waived in accordance and compliance with Rule 2.11(C) of the Model Code.The end result seems to be based on common sense, but it does not quite provide a way to deal with the tough cases since it seems to amount to saying that "it depends" and that it will be the judge who decides how much it depends. The opinion also does not address an issue that had been in the news not too long ago regarding "facebook friendships." For more on that go here.
You can read the opinion (No. 488) here.
UPDATE 10/24/2019: Legal Ethics in Motion has posted a short comment on the opinion here.