*Disclaimer* I mean NO hate by the following, and I am talking about skis that is used primarily if not solely in the park.
For everyone who says "Dont use a file", watch this video and remember these are PARK skis, you are doing what the rail is going to do to them anyways, only you're controlling the pressure and where on the ski it's getting detuned. Now, here is a video on how to tune/detune your skis, please pay attention at around the 5:00 mark and you will understand why a file is good to use:
I agree that a gummy stone is a good way to go, but it's not going to help round out the edges underfoot, and rounding out the edges underfoot can help prevent damage to the edge. This is not something to do to an all-mountain/jump ski, but most likely he's planning on using them primarily in the park, and park skis get thrashed hard, but by detuning the edges properly you can lengthen the life of your ski.