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.
It is always difficult to decide whether it will be good to hold the cryptocurrency or sell it. Therefore, I think that it is senseless to regret about the things that were already done. Simply wait when the crypto starts going up.