It depends a bit on whether they're prepared to move. If they are then it makes sense to learn a niche thing where demand outstrips supply. If they don't live i

It depends a bit on whether they're prepared to move. If they are then it makes sense to learn a niche thing where demand outstrips supply. If they don't live in a big city and need to find work locally it probably makes more sense to learn something popular, or where there is a known local demand.