Tente adicionar "friends:true" assim:
db.getCollection("users").find(new BasicDBObject(), new BasicDBObject("friends", new BasicDBObject("$slice", N)).append("friends", true)).next();
Deve ter os campos _id e friends.
db.getCollection("users").find(new BasicDBObject(), new BasicDBObject("friends", new BasicDBObject("$slice", N)).append("friends", true)).next();