Hmm, as a huge fan of older tools I would be tempted to find a nice set of older chisels. The steel used in them is far better than a lot of the modern equivalents. I would personally be happier with a set of older ones than a single new (and really expensive!) one. The set will give your new woodworker a range of choices learning that a single item simply will not.
As an aside, some of the sets available at a moderate price are actually very good providing a little preparation work is carried out on them. I realise that Lie-Neilson is a fantastic product but I think it would make a little more sense to learn with a more moderate tool.
Once a “feel” for the use of a chisel has been gained and a bit more experience then a premium chisel would seem to make more sense.
If I had to choose just one it would be a 1/2″ bench chisel. The reasoning is that the majority of woodworking lumber seems to be 3/4″ or thereabouts and a 1/2″ chisel gives enough area to pare without spanning the entire width. On the other hand to chop out mortices you would need a 1/4″………its never simple is it?
This is just my personal opinion…….