I think this is where I'd set the requirement, rather than requiring writing code.
If they can make (and talk through) a mockup that demonstrates a grasp of the problems of real-life UI/UX design -- including the little details that normally just get hand-waved over, like "when & where exactly will we get all of the data that's required to build this screen?" -- that counts for quite a lot.
Writing code for someone who doesn't have the background has a huge learning curve -- if someone sets them down with the exact language(s), libraries, editors, references, etc. to use, it's not so bad, but someone who doesn't know the landscape will go down a lot of very time-consuming dead ends on their own, and the lessons they'll learn will very likely not be worth the time wasted (assuming they have no plans to change careers...).
If they can make (and talk through) a mockup that demonstrates a grasp of the problems of real-life UI/UX design -- including the little details that normally just get hand-waved over, like "when & where exactly will we get all of the data that's required to build this screen?" -- that counts for quite a lot.
Writing code for someone who doesn't have the background has a huge learning curve -- if someone sets them down with the exact language(s), libraries, editors, references, etc. to use, it's not so bad, but someone who doesn't know the landscape will go down a lot of very time-consuming dead ends on their own, and the lessons they'll learn will very likely not be worth the time wasted (assuming they have no plans to change careers...).