Let's say an elf is making a perception check at night. A human decides to try and assist the elf. The elf has dark vision, while the human does not. Can the human even help the elf? Because if the human was making the check himself he'd have disadvantage because it was night. Does the human give the elf anything extra if he himself has almost no ability to see in the dark?
[RPG] Can a human assist on darkness perception checks
dnd-5eelfhelpinghumanvision-and-light
Related Solutions
This is how skills are supposed to work!
If you are in a situation where there is only one person doing something, and they are rolling a single skill check, then yes, this is how it's supposed to work. Giving help is a natural thing and should be used in situations like this. There is no reason to prevent it unless the task is clearly something that's not going to benefit from someone else giving you assistance. There are some things you can do to limit it.
It's also worth noting that helping can often save you some table time. As a AceCalhoun points out in the comments, in many cases what happens if you don't help is that everyone in the party tries their hand at the task. This behaves very much like advantage, but with a slightly lower overall modifier (because most likely you'll have one character who is good at a task and the rest that are lower). So Working together only raises the change of success slightly and consumes less table time in these cases.
Be a bit more stringent about what you allow for assistance. Is coaching stealth really all that helpful? do you really want someone looking/talking over your shoulder while you're picking that lock? Evaluate situations where characters attempt to aid more carefully.
Have more than one thing going on at once. If all the characters need to be stealthy, they can't be helping each other. And if you need two arcane characters working on the sigils on opposite sides of the room, maybe they have to choose which one gets help from the third (or don't have anyone to help at all).
Make things take multiple rolls and limit helping on all of them. Maybe the first roll the wizard can be helped, but after that he's on the other side of the trap, or arm deep in the sigil or something to where additional assistance isn't going to help him.
Figure out how to inflict disadvantage for the task. Maybe there are mitigating circumstances.
Create a distraction. A rogue can't help the wizard if he's busy fighting baddies. Make some skill checks happen in an occupied room. Make completing the skill checks the win condition rather than defeating the enemies.
The basic crux of all of this is that helping is supposed to a mundane task that provides advantage. Yes, that's a huge deal, but it also doesn't stack with other things that give you advantage and it can be easily cancelled by disadvantage.
So get creative! Build some situations into your adventures that prevent your heroes from helping each other (or make the opportunity cost higher). But don't do it all the time, that might get tiresome. Adventurers like to help each other out, let them, but don't make it easy all the time.
The extraordinary ability low-light vision is a mess...
The elf racial trait low-light vision says
An elf can see twice as far as a human in starlight, moonlight, torchlight, and similar conditions of poor illumination. (PH 16)
The Player's Handbook on Vision and Light adds that one should
Double the effective radius of bright light and of shadowy illumination for such characters [that possess low-light vision]. (165)
The Monster Manual provides a similar description of low-light vision that's slightly more explicit:
A creature with low-light vision can see twice as far as a human in starlight, moonlight, torchlight, and similar conditions of shadowy illumination. (311)
And the Dungeon Master's Guide provides a description of low-light vision:
Characters with low-light vision have eyes that are so sensitive to light that they can see twice as far as normal in dim light. [...] Characters with low-light vision can see outdoors on a moonlit night as well as they can during the day.
Finally, the Rules Compendium provides another description of low-light vision:
Creatures that have low-light vision can see twice as far as normal in dim light. ... Those that have low-light vision can see outdoors on a moonlit night as well as a human can during the day. (115)
In other words: Yuck, or Ask the DM.
...But you can probably safely just take the best parts
The light levels poor and dim aren't listed Table 9–7: Light Sources and Illumination (PH 165), but shadowy totally is. And while torchlight can be either shadowy or bright illumination according to the same table, moonlight and starlight aren't even on the table.
So from the Player's Handbook section on Vision and Light take double the effective radius of bright light and shadowy illumination, from the Monster Manual description take starlight, and from the Dungeon Master's Guide description take see outdoors on a moonlit night as well as they can during the day. Put them in a Word Blender. Pour out...
Low-light Vision (Ex): A creature with low-light vision can see twice as far in bright and shadowy illumination. It can see outdoors on a starlit or moonlit night as well as a human can during the day.
When the torches go out (or whenever the DM says the night's overcast), such a creature is still blind, which is the rather severe downside of low-light vision. (I'm not sure discussing low-light vision with regards to flaws is entirely fair, given that one can always mitigate one's flaws simply by playing differently.)
"But how far can a human see in moonlight?"
Effectively, creatures can see as far as one of us humans actually can... that is, until the creature needs to use the skill Spot to determine the location of enemies.
The skill Spot isn't used to determine the literal distance a creature can see but how far away a creature can detect enemies. Each terrain has a Stealth and Detection entry that's used when determining the distance at which encounters effectively begin. For example, Stealth and Detection in a Forest says
In a sparse forest, the maximum distance at which a Spot check for detecting the nearby presence of others can succeed is 3d6×10 feet. In a medium forest, this distance is 2d8×10 feet, and in a dense forest it is 2d6×10 feet. (DMG 87)
So the DM determines the distance randomly. Then according to the skill Spot
The Dungeon Master may call for Spot checks to determine the distance at which an encounter begins. A penalty applies on such checks, depending on the distance between the two individuals or groups [a −1 penalty per 10 ft.], and an additional penalty may apply if the character making the Spot check is distracted (not concentrating on being observant) [a −5 penalty]. (83)
But the DM needn't call for such Spot skill checks and just declare the encounter begins. Further, if one or more creatures are in shadowy illumination those creature have concealment and can use that to make Hide skill checks, probably substantially reducing the chances they'll be noticed by an unobservant sentry making a Spot skill check. If one or more creature is in darkness then I hope you like rules.
D&D 3.5 never says exactly how far folks can see. Instead, the game explains how far away folks can see the next pile of XP.
Related Topic
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Best Answer
The Working Together rule suggests that the human won't be much help
... if the perception check relies purely on sight. For that case, it's difficult to argue that the human can be of help to the elf. Granted, the human could try alone at disadvantage, so this doesn't explicitly forbid this attempt to detect whatever's out there, but remember that ...
Perception relies on multiple senses
If the human uses other senses such as hearing/smell/touch, etc, there should be situations where their help would warrant giving the elf advantage under the Working Together rule.
Perception isn't confined to sight
Some animals get advantage on perception checks via smell, some with hearing, if you want to see rules based precedent for perception being a multi-sense ability.
Bottom Line: what's the situation, and what are they trying to detect?
If the detection attempt isn't purely visual, there's a solid argument for the human being able to apply "working together" rules to give the elf advantage on the check.