I'd suggest against buying. At 5'6" 120lbs, he's almost guaranteed to put on a good bit of weight in the next couple years, and probably a few inches as well. That will make any ski that works for him now too small and soft soon. And with a budget of $400, you're going to be so limited in your choices to start, that it's not really worth the expenditure.
As far as freestyle skis, forget them. I'm sure your friend's son wants to be a park rat, and do rails and hits and the pipe. Unlike most others on this forum, I have been a park skier. Freestyle skis are designed to perform well in the park. They are not designed to perform well for a beginner. Besides which, you need to emphasize to him that he needs to be an advanced skier before he should start doing anything that would require a park-designed twin tip. He will get a lot more respect from park skiers such as myself by developing his skills than by slapping on a pair of 4 year old Rossi Scratches. I don't mean to be disrespectful towards this kid, but I feel the need to be mercilessly blunt when presented with a situation like this. A kid who is a beginner now will not have the skills to be in a full-blown park for at least 2 years, most likely more, since he probably won't be getting more than 20 days on snow per year. And the window of opportunity for skiing park is actually fairly short. I'm still short of 30, and most of my friends and I have moved away from park as the primary focus of our skiing. It's just too rough on a person's body to do for too long, it's something for teenagers and college kids. Being a beginner at 15, he's most likely started far too late to develop park skills in a time span where his body will tolerate it.
With what he seems to want, and his budget, tell him to snowboard. The progression into park on a board is shorter, and decent equipment is typically a bit less expensive.