Grow his portfolio? You can grow your Portfolio by buying low and selling high when the price appreciates and most of the time, it takes time to sell at the high time except if your relative is a trader and being a trader can be a daily or occasional trader who trades when in the mood or see an opportunity.
However, can one buy low and sell high and still increase their portfolio? You can increase your portfolio when you buy coins more and more with dollar cost average in every opportunity which is always this kind of times(bear time) and sells high when the price appreciates.