It's up to the DM
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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:

1. The DM describes the environment. 
2. The players describe what they want to do.
3. 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 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
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> 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. The DM uses advantage and disadvantage 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 helping another, *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.