As in he should in every 4 years be taking 50% of whatever total amount plus profit till it gets to the 16years he planned. At least in worst case scenario this method could save him something reliable to fall back onto.
Taking 50% where? I don't understand what you mean.
Do you mean selling 50% of the investment every 4 years? What is he supposed to do with the money? Wait for a bear market to buy more bitcoin? It's a retirement plan, so he probably wants the money to remain not taxed until he starts spending it and he doesn't want to pay exchange fees every few years. Your strategy would work only if bitcoin followed 4 year cycles for the next 3 cycles.
I believe that he's meaning to sell 50% of the acquired profits, if there are any, every 4 years. By doing so, he would be decreasing the risk of ultimately losing his initial investment if things take a sudden turn for the worse because he'd be claiming profit throughout the 16 years of the investment. However, there isn't any guarantee that in those four periods of four years, there'd be a profit in between to claim; we could be suffering another bear market that could last years, similarly to the one we're going through now, or in 2018–2020.
I'm generally confident in myself regarding Bitcoin; I don't believe that it'll burst in the upcoming years, but 16 years isn't a short time period. Throughout all these years, the market would have had an abundance of ups and downs that could lead to a possible sale or the abandonment of the initial plan.