The answer depends on where your friend is based. I will cover the situation from the UK legal perspective, but if he is in the USA, then the law is slightly different there, both in terms of what is protected in the first place and the degree of freedom to make copies under the fair use doctrine. Youtube operates under US law and consequently acts upon DMCA takedown notices, so that may be more relevant in this case.
However the remainder of this reply just looks at the UK law.
It is worth mentioning that I am not familiar with the Lord of the Rings films and so I don't know how Hornburg castle was created on screen. I assume it was not based on a real location, but if it was a real building then there would be no infringement in re-creating it as a model because it is safe to assume that the building is old enough to no longer be covered by copyright. However for the other alternatives such as a purpose-built set or if the scene was created using CGI or a matte screen etc copyright will probably exist in the rendering of the castle as an artistic work. However the way that work has been recorded is in the context of the movie as a whole (rather than, say, any design drawings) and so whether or not the castle amounts to a substantial part of the overall film is the main question when it comes to determining whether any copying it amounts to infringement. This applies even though what is being considered is a three dimensional copy of a two dimensional work (see section 17(3)
). The UK courts are less willing (compared to the US courts) to class individual plot elements or minor characters etc as worthy of copyright on their own and the same would hold for scenes within a film, unless they are clearly very central to the story, and hence form a substantial part of the film. That said, the scenery of a play has been held to be something which can be infringed by copying it. Copyright protection usually becomes more viable where there is an accumulation of elements each of which is neither commonplace nor unoriginal. So, not knowing how central the location of the Hornburg is to essence of the film, it's hard to say whether the courts might find it to be a substantial part of the overal copyright work, namely the film(s) in which it features. Let's assume for a moment that it is a substantial part.
The next step is to look at whether there are any exceptions to the general rule which mght permit the teaching of model making as fair dealing. Unfortunately none of the educational exceptions
would apply either because of the commercial nature of the instruction or because instructional videos of the sort you have described do not meet the fairly narrow definition of an educatioanal establishment.
Similarly the fair dealing exception for private study or research also fails because of the commercial nature of the venture, and in any case I think the purpose falls outside the 'research' criterion. None of the other exceptions are applicable.
On that basis, it seems possible that under UK law there may copyright protection for the visual representation of The Hornburg, assuming that a subjective test indicates that it forms a substantial part of at least one of the films, and there don't appear to be any relevant exceptions to allow for an instructional video on how to make a model if it. But equally important, given that your friend is obviously keen to avoid hassle, is the question of the attitude of the owner(s) of the copyright in the film(s), and thus the likelihood that the film company will take issue with the model making. There has been a series of court cases over copying a simple stormtrooper's helmet from the Star Wars films, and so one should not underestimate the pettiness of the film companies when it comes to protecting their IP. Moreover, if there is a merchandising opportunity for a toy company to sell models of Hornburg Castle, this increases the likelihood of a proactive stance by the film company. A quick bit of research on Google indicates such toys and games are available although whether they are all licensed merchandise is less clear. I also note that there appear to be a number of videos on Youtube relating to models of Helms Deep, which tends to suggest that the film company isn't too bothered about this sort of activity, assuming the video makers haven't sought permission first. Overall though, and unfortunately for your friend, the Saul Zaentz Company which owns the rights in some of the Lord of the Rings films has something of a reputation for its robust approach to protecting its IP. (see here
I'm not sure that helps you and your friend. But at least it gives some indication about the law as it stands in the UK.