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.
Well many people believed in holding when the market was high for they dreamed of much higher prices in future but it take a moment for the market to fall to the ground that is why it is called uncertain market. It is useless to cry over spilt milk right now and one should rather look ahead to make high future prosperous my investing again in a good coin and holding till the time it can give away the maximum possible price.