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.