Is DM allowed to fudge dice in Adventurer League play?

Question

You're Empowered. Make decisions about how the group interacts with the adventure; adjusting or improvising is encouraged, so long as you maintain the adventure's spirit. This doesn't allow you to implement house rules or change those of the D&D Adventurers League, however; they should be consistent in this regard. AL DMG, pg. 5

Does this mean the DM may fudge a success/fail ability check/attack roll/saving throw?

Usually I'd say yes, but then if a PC is failing his death saves three times, and the DM say that he counts that as save/neutral, I feel a little bit uncomfortable.

Answer 1

TL;DR: it's bad if you are using it to cheat, what exactly is considered cheating is up to DM's discretion, although fudging dice rolls is explicitly called out in the Cheating section (p. 6 from DDALDMG).

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It isn't explicitly stated anywhere in the rules that it is allowed or forbidden. My answer is actually that it depends on the intention, as your question notes.

Technically, yes.

I will state my case. First, the DM is allowed to change the encounter itself. Wall of text ahead about changing the encounter. It's stated the section before the quote you made.

While the adventure provides suggestions on how to adjust an encounter to provide an appropriate challenge for your players, they are just that—suggestions. You may, at your discretion, make other adjustments to the encounter by adding or removing monsters. While the monsters you add may be different from those listed in the encounter or the sidebar, they should be thematically similar. For example, if your players are encountering a group of zombies, adding a bunch of pixies doesn’t make much sense. However, adding a zombie or a ghoul might. Keep in mind that while the characters earn XP for these new monsters, the maximum amount of XP they earn for the adventure does not change. Remember to give them a challenge, but don’t make the adventure unbeatable.

In sequence

Challenge Your Players. Gauge the experience level of your players (not the characters), try to feel out (or ask) what they like in a game, and attempt to deliver the experience they’re after. Everyone should have the opportunity to shine.

and

Keep the Adventure Moving. When the game starts to get bogged down, feel free to provide hints and clues to your players so they can attempt to solve puzzles, engage in combat, and roleplay interactions without getting too frustrated over a lack of information. This gives players “little victories” for figuring out good choices from clues. Watch for stalling—play loses momentum when this happens. At the same time, make sure that the players don’t finish too early; provide them with a full play experience.

Note that the DM gets permission to do a lot of things, if these are for the goal of making a better adventure. Fudging rolls, while not the best technique for adjusting things, is one. For example, you might want to fudge rolls to make up for the fact that you put too many monsters and created a too hard challenge - or you might want to fudge a critical when the encounter ended up too easy and you want to deliver a little more damage - these could be the replacement for removing or putting a monster, which you are allowed to do. If you are doing it to make the adventure better, it should be fine.

However

If you are doing it to "cheat" (or allowing cheating), you are not fine. The DDAL DM Guide has a section about cheating - usually about players cheating, but we should extend that to the DM. AL is based on honor system, so nobody is going to be constantly checking on you or whatever, but note that fudging dice rolls is explicitly called out there.

If you notice something amiss—either with the paperwork or during the game (fudging dice rolls, not checking off used items, and so on)—discuss it with the player and attempt to resolve irregularities. [...] You can ask a player to reroll a dice roll that isn’t obvious to the table.

Conclusion

So, as you yourself mentioned in the question, sometimes it's fine, sometimes it's not. Did the PC die in the encounter fair and square, in a balanced encounter, possibly due to his own bad decisions? Then yeah, I would call fudging his rolls cheating. Did the PC die because you mistakenly put 2 monsters more than you should have and the encounter got way too hard? Yeah, let's pretend that 2 was a 20.

Answer 2

While this isn't "fudging" you have some leeway with d20 rolls

As a DM, you are able to apply advantage or disadvantage to a given roll based on the situation/circumstances.

Applying advantage during play (Chapter 7)

The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result. (Basic Rules p. 57, / PHB p. 173)

The point is that unlike the classic fudge ("Hey, the die fell off the table, roll again!") advantage needs to be established before the roll, not after a failure. If you want to stretch this into a means to DM fudge I would strongly suggest that you use with care. There needs to be a narratively plausible "here is why there is advantage/disadvantage" explanation, otherwise the players may begin to expect or ask for advantage if they see it offered with little in-world justification. In an Adventurer's League setting, if some of the players see this applied with an odor of favoritism or blatant fudging there may be a report to the event coordinator regarding the DM engaging in foul play.

It's not a fudge

The above approach isn't cheating, but is very much Rules As Written. What it hinges upon is DM judgment and timing. Applying circumstantial advantage requires some foresight/forethought before applying it.

• At table experience: I have found in the games that I have been in that it needs to fit the narrative or it becomes a meta/thing that smacks of either DM favoritism or something else.

While there isn't any explicit restriction on when a DM can apply advantage or disadvantage, I'd recommend that you treat this like a silver bullet in the context of this question: you only have a few of them in your gun belt, so apply some foresight, expend them with care, and make sure that it fits the narrative/situation.

Answer 3

Bad luck happens to everyone and anyone can have one of those sessions where the dice will just not roll for you.

In that case I would argue GM discretion would allow them to "deus ex machina" the encounter and, if need be, the rolls.

Additional Tips for the Dungeon Master
As the Dungeon Master, the most important aspect of your role is facilitating the enjoyment of the game for the players. You help guide the narrative and bring the words on the pages of the adventure to life. The outcome of a fun game session often creates stories that live well beyond the play experience at the table. Always follow this golden rule when you DM for a group: Make decisions and adjudications that enhance the fun of the adventure when possible. To reinforce this golden rule, keep in mind the following. (p. 5 AL_DMG_SKT.pdf; italics added)

It shouldn't be a regular thing, GMs shouldn't be "fixing" the game so their friends have an easier time but certainly I wouldn't consider it cheating if the rationale behind it is to balance out a bit of bad luck. As HellSaint said; if it's the players bad decisions then tough, should have made better decisions. Just can't roll higher than X, even though your character should be able to do this with their eyes shut and hands tied behind their back? You passed, don't tell anyone...