Can a judge be guilty of giving Outside Assistance?
To put things plainly:
A judge–in the course of doing his or her judge duties–cannot be guilty of Outside Assistance.
What they can be guilty of is bad form and potentially form so terrible as to hurt the perception of the integrity and competence of judges.
If a judge walks by a match and randomly blurts out, “Golly, block or die, Mr. Davis,” then they're providing Outside Assistance. If Mr. Davis asks a judge a rules question wherein the judge has to explain that the result is lethal, that's fine. The sticky part is not in finding the line between Outside Assistance and bad judge form (unsolicited comments or comments not related to a judge question are OA, dutiful judge comments that go too far are bad form). The sticky part is the line between a “good” judge answer and a “bad” judge answer–even though both might be “correct.”
I find the best way to determine the line between good and bad form is to look for one of three options:
1) Is the answer a rules quote? (Exile is not a destroy effect. While resolving a spell, follow the instructions in the order written.)
2) Is the answer and unambiguous question leading to a Yes / No? (Yes, it is dead. Yes, that can target that.)
3) Is the answer and unambiguous question leading to a specific scenario? (Until the spell with split second resolves, you can't activate or play anything, but that trigger still happens and stacks as normal.)
The key element that goes with this is to answer the question that was asked, not the question that the player meant to ask. If a player asks the classic Spellskite / Lava Spike question (can I target Lava Spike with Spellskite?), there's a unambiguous answer. It's bad form to correct the player's question or fish for a different question. (At Competitive that is; at Regular, educate over enforce, please.) If a player asks an ambiguous version, then you might have to clarify (can I Spellskite that Spike?), but don't fish–just say “please clarify” or “your question is unclear, could you clarify?”
Where judging becomes more art and less science is in how you answer that awkward question. I find that for questions where the player is asking a legitimate question but not what they think they're asking, a strict rules comment followed by me pausing at the match is generally best. ~Can I target Lava Spike with Spellskite? Yes, the spell is a legal target for the Spellskite ability.~ Players that have some clue but are a little lost will often wonder at the rules-y reply and ask a follow-up that actually gets to what they actually want. Players that are lost will just go on and smack into reality soon after. Either way, the pause means I'm right there as things happen and can provide a follow-up answer if a player asks or make sure things resolve correctly if they missed the difference between what they asked and wanted to ask. When they go on with suboptimal play and look to me with confusion, being there allows me to immediately address how the player's query was not what he wanted and talk about Competitive REL and how it's frowned upon for judges to correct an otherwise legal question.
Some judges may feel this is overly harsh. At Regular, you are likely right. At Competitive, you are wrong. Conveniently, our rules already address this. But why does that strict delineation exist at Competitive? Won't that player be upset? Yes, that player will be upset, but matches aren't one player. If you coach the player out of their ignorance, and the opponent is upset because you assisted the player. What might seem “nice” is actually taking away the competitive advantage players expect for better rules knowledge, and it directly hurts another player. Thus, for Regular I advise being nice but for Competitive, keep the advantage. I often find that using cases like this to illustrate for players how expectations are different for players and how judges operate differently at Regular and Competitive goes a long way to smoothing out the upset a confused player may feel. If opponents at Competitive were little angels looking out for some righteous ideal, they have every right to stop and ask the judge a follow up question just as much as the player does. Every once in a while I get an opponent who does, and I happily answer them. More often than not though, I get an opponent who obviously appreciates having a judge who sticks to what was asked without digging around–especially when that judge hangs out for a moment knowing they're about to get called again (showing they knew the question wasn't what the player wanted even if it was what they asked).
The classic example* with sample responses:
Can I target Lava Spike with Spellskite?
Regular REL: I suspect your question isn't asking what you want to know, but at Regular REL, please allow me to clarify. The Spike is a legal target for the ability, but the ability can make the Spellskite a legal target for the Spike, so it won't ~do~ anything.
Competitive REL Good Form: Yes, the spell is a legal target for the Spellskite ability.
Competitive REL Bad Form: Yes, the ability can target the spell. Did you want to know more?
Competitive REL Atrocious, Reputation Damaging Form: Yes, the spell is a legal target for the Spellskite ability, but the ability won't do anything.
* For all you young folks out there, the actual classic example would use Misdirection and either Counterspell or Duress.