In a way similar to how a Wizard can copy spells from another Wizard's spellbook, can a Druid learn Wild Shape forms from another Druid?
Rules as written: no.
The rule is that you have to have seen the beast you want to take the form of. Having that creature described to you, or drawn for you, or whatever else does not meet that. There's definitely some ambiguity left in the feature overall (see Can a druid shapechange into the form of a specific, individual beast?), but I think your question is clear (and the answer is equally clear: "nope").
If we're talking about the other druid actually changing form to show how it's done, that's probably also no by RAW, but there's ambiguity. The specific rule is:
you can use your action to magically assume the shape of a beast that you have seen before.
and if we parse that like a computer, that probably is
assume the shape of a beast [that] you have seen before
but it could be
assume the shape of a beast [that] you have seen before
A wild-shaped druid is a druid in beast form, not actually that beast. The first (and I think more obvious) reading is that you need to see the actual beast — but it could actually be that seeing the shape is adequate. But, 5E isn't meant to be subjected to fine-grained logical parsing. As with Can Polymorph be used to give a druid new Wild Shape forms?, this is ambiguous and ultimately the DM's call.
From a rules-as-intended point of view, I think it's pretty clear that the idea is that you should have direct experience of that animal — but that it's not really meant to be strict. Rather, it's meant to make sure the druid conforms to the story-based concept of the class, as a guardian of nature with connection to the land and to a specific environment — not like a wizard shape-shifter showdown.
The Xanathar's Guide optional rules suggest that even without going out of your way to collect special forms, you've "probably seen" basically every animal appropriate for the environment you're in by 2nd level. The obvious intention is that you can shift into animals reasonable for your environment and history, not that you have some mental menagerie which is somehow equivalent a wizard's spellbook. In fact, it's instructional that there is an Unearthed Arcana optional rule which does work like collecting spells, but that didn't make it into the official rules and instead we get the much broader guidance.
Now, from a game balance perspective, is there any harm? Probably not, as long as you're sticking to from-the-rules stat blocks, or at least creatures which are reasonable as beasts, not some sort of hybrid fantasy creature with superpowers which someone has homebrewed and labeled a beast for the purposes of overcharging the druid feature. And I'd go so far as to say that the class has a lot of weird holes and gaps at various levels if you play this restriction too strictly. It's more fun to have options, and the rules support that — see this answer on filling in at CR levels where the Monster Manual doesn't have an entry that fits your environment and backstory.
All the druid needs is to observe the animal in question, so another druid using wildshape to show the animal shouldn't be a problem. It wouldn't be out of line to say it takes some hours to get enough observation of how that animal moves and acts to be able to wildshape, though (similar to the time needed to copy a spell).
It isn't clear what 'seeing' a creature really means. Is it more spiritual, watching the beast to find out how it lives, what its habits and natural behaviors are, to take in the fullness of the creature's existence (and can you even do that in a few hours, or even days)? Or are you watching from a more mechanical standpoint, to see how its body fits together and operates as it moves and climbs and hunts? Or is it a mere glimpse through the trees? One of those, you can't get from a wildshaped druid, but the other two, you certainly can. It's up to the DM and player to figure that out at the table.
But I honestly don't see anything problematic about sharing wildshapes. The point of that restriction is, to my mind, about giving the DM a reasonable way to restrict exotic or unthemely forms. ('Have you ever seen a camel, here in Fantasy Wales? And why would you even know what a triceratops IS?') Making a druid go off on a side-quest just to learn how to be a seal seems punitive and most likely doesn't add anything to your game except busy-work.
The DM still can make reasonable rulings about what forms another druid knows, since they're likely to not be of the relatively high level required to learn really dangerous combat forms (if they even can; they may not be Moon Circle), and whether they're even willing to spend most of the day showing off local animal forms to some dude from out of town.
Overall, it gives the druid an extra tool in the toolbox, but doesn't really remove any of the DM's control over what forms you might know, so I don't see any strong reason to say a wildshaped druid wouldn't count for observation.
Yes, as implied by a similar answer from Jeremy Crawford.
Jeremy Crawford was asked:
[Is it true a] druid can cast a spell to conjure animals and then transform [into] the animal he only saw [via] this magic with wild shape?
To which he replied:
Wild Shape requires a druid to see a beast before being able to transform into it. The feature doesn't specify the means used to see.
Conjure Animals creates Fey rather than true beasts...yet the druid still learns what is needed to Wild Shape.
This means that if seeing transformed Fey are enough to learn to Wild Shape into a beast then a druid watching a Wild Shaped druid can presumably learn enough for themselves as well.
Yes, as implied by the ability itself.
Wild Shape says "you can use your action to magically assume the shape of a beast." Wild Shape specifies that the Druid assumes the shape rather than the form or essence or other aspects of a beast. The ability bypasses specifying the Druid even needs to see the beast in action. This also fits Crawford's requirement that the beast merely needs to be seen.
This implies shape is enough for a Druid to figure out how a creature works. This fits the idea there are only so many ways to use claws, beaks, teeth and other forms of attack and motion - and a Druid merely needs to see a creature to get a sense for it.
This also fits the idea that if one druid studied an animal long enough and learned enough to Wild Shape into it then a druid seeing that Druid in action via Wild Shape has seen the essence of what is needed to transform themselves.