At 5'7" a 171 would be at his head, and would be the proper size. He needs to realize that these skis will be mounted at or very close to center, and something around 160 is absolutely tiny.
If you want to learn the hard way why a short park ski is a bad idea, go for it. But as several knowledgable members already said, that's way too short.
Get an AR7 in the 170 range, or get a 160ish ski and land on your face with your first frontseat 180.