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.