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.
That's why taking advantage of opportunities is the most important thing in bitcoin investment, sometimes greedy and waiting for higher prices will hmake you lose very much. but everything has passed and we have to look forward. very possible price changes like last year, don't panic and don't despair..