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?
The Working Together rule suggests that the human won't be much help
A character can only provide help if the task is one that he or she could attempt alone. (PHB p. 175)
... 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
Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings, and the keenness of your senses. (PHB, p. 178)
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.
Sometimes, two or more characters team up to attempt a task. The character who's leading the effort -- or the one with the highest ability modifier -- can make an ability check with advantage, reflecting the help provided by the other characters. (PHB, p. 175)
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.
- Examples (PHB, p. 311)
Tiger: The tiger has advantage on Wisdom (perception) checks that rely on smell.
Wolf: The wolf has advantage on Wisdom (perception) checks that rely on hearing or smell.
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.
No, because elves can't make perception checks
All joking aside, checks are what the people at the table do to model uncertain outcomes of what their characters are doing.
So, what is the elf trying to do? Perhaps stand on watch, alert for intruders?
So the real question is: How could a human help in this situation?
The answer: Stand watch as well. Two sets of eyes and ears are better than one (even if one of them is pointed :-).
Even though humans do not have darkvision, they can still see a little at night. They can, of course, still hear and smell the approach of intruders. So, they can help in this activity.
Gamewise, I'd probably make the elf's player and the human player both roll Wisdom\Perception checks, perhaps at disadvantage for the human, rather than allowing the elf's player to make one check at advantage. Either way, two rolls are made to detect incoming threats, with only one of them needing to overcome the intruders' stealthiness.
The basic rules for Helping another character grant advantage on the check. Simple enough in combat - Character A does something to distract Target B, granting advantage to Character C.
However, Working Together goes into more detail.
A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves' tools, so a character who lacks that proficiency can't help another character in that task.
Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help.
There's an element of DM discretion here.
Working together involves guidance or assistance. For example, in picking a lock1, the second character would be serving as extra hands - holding one tool in place while the first character manipulated a different tool, or describing what he sees inside the trap in a situation where you can't view the interior and work the exterior at the same time.
Detecting an ambush, standing guard duty, and spotting traps are all instantaneous functions, and very much pass/fail - you either catch a glimpse good enough to identify the threat, or you don't. I would not allow one character to help another with that.
If the object being observed were off in the distance, I would allow one character to help another, but only after the first character was successful on his own. If there is no time issue, I would have no problem with "Look over there. No, higher. Not the whichamacallit. Yeah, the whoziwhatzit, above the thingamabob. See it now?"
1In the case of thieves tools, I'd require both characters to have proficiency. They need a common lexicon and similar basic skillset to work together. They both need an idea what the other is doing or thinking, even if one is more skilled than the other.
It's up to the DM
The situation you ask depends on the circumstances, so it's a matter of DM's ruling.
Let's say an elf is making a perception check at night. A human decides to try and assist the elf.
You're putting the cart before the horse. The elf player is making a perception check. That means the player has already announced his actions. As the result, the DM asks him "make a perception check". Something has already happened in the world, and now we are finding out the result. Another player can not retroactively change this by interrupting the roll and saying "I assist".
Players aren't supposed to announce "I am making a perception check" and instantly roll the dice. They announce their character's actions. It's the DM who is supposed to choose an appropriate roll and resolve the outcome. See "How to play", PH page 6:
- The DM describes the environment.
- The players describe what they want to do.
- The DM narrates the results of the adventurers' actions.
For instance, the elf player says "I'm going to be standing watch all night". The other player says "I'm not tired, so I'll go with him. It's quite dark tonight, but I hope my ears still help". Then the DM says "Okay, maybe you've noticed something, let's see — make a perception check".
The DM can also add "...with advantage, since your ally helped you". He can also neglect the advantage, deciding the sight is crucial in this particular situation, and human's hearing don't help the elf to see better:
a character can help only when two or more individuals working together would actually be productive
Fiction first here, not mechanics. "Actually be productive" isn't a game term, it's more about in-world situation, hence, up to the DM.
Moreover, the DM can choose to ask for a Group Check instead (see PH, page 175). Or use Passive Perception. Or ask for two separate checks. Or not to ask for any check at all. It depends both on the particular situation and the DM's gaming style.
Advantage and disadvantage are meant to be the DM's tools
Can the human even help the elf? Because if the human was making the check himself he'd have disadvantage because it was night.
Advantage and disadvantage are game mechanics, which go from the narrative, not vice versa. The DM uses this mechanics to reflect what's happening in the world.
Instead of thinking "A PC rolls with disadvantage therefore he can not help other PC", think the opposite: "One PC are effectively helping another one, therefore roll with advantage".
Of course, there are mechanical features which explicitly says they impose (dis)advantage. But the DM can freely ask for a roll with (dis)advantage, depends on the situation, see PH, page 173:
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.