This is a tricky one, but I lean towards just answering the question in good faith and add a comment warning about the possible copyright infringement.
First of all, recreating a copyrighted design for personal use isn't really illegal (or it doesn't matter if it is because no one will ever know). The legal problems arise if the copied design is distributed. Nothing wrong in copying some copyrighted design just for the sake of it, as long as the result is never shared with the world.
Secondly, recreating old designs is a part of the job of a graphic designer. A client could hire you to recreate some old logo, poster, drawing etc. If the client holds the copyright of the original, there is nothing wrong in recreating it. The involved disciplines could in my opinion be very suitable subjects for questions on this site.
In reality we can never really be sure if the OP is telling the truth. Any question about image editing could be met with an "are you sure you are allowed to use that image?" and the OP could simply answer "yes" and we would have to believe it.
In the given example we can't really know if the OP was actually hired by the person who holds the copyright of the posters. It isn't 100% clear that something illegal is going on, so I would give them the benefit of the doubt, but of course mention the copyright issue.
We've had other questions where it's much more obvious that answering the question would actually help the OP break the law. Questions like "How do I make t-shirts with Disney characters?", "How do I bypass this copy protection?" and similar, which clearly don't even come from the desire to learn a technique, but more from a desire to make quick money stealing, shouldn't be encouraged.