There is no sense in blaming yourself for not selling in December. You won't reverse the time and you could not predict what will happen.
Your advice is correct, he does not need to blame himself. If everyone knows before the price, they will easily get rich not only with bitcoin but also with other coins.
This is where people fail to understand that it is market and not everyone can sell at the peak and no body exactly will know the peak else in the world everyone would be rich and no body would be cribbing to have more money. So if this is understood life becomes more sorted.
But it is a basic rule of trading, that people think that one day they will see bitcoin in a peak and then they will sell all bitcoins and make rich and later on when bitcoin will dump, so once again they will buy it.