Beginning of January would have been indeed a good time to sell and make a break. But this was anyway an exceptional downswing and recently the market has recovered. if you have such a huge amount avalable, I would leave the already high priced coins and try my luck again with several low priced altcoins. Alternatively and depending on your skills you could also start your own ICO project.
If he made that much and sold back in december or yeah even in january, that was the best time to quit or withdraw a substantial amount of it. The best action would have been to convert to fiat then buy back bitcoins or the alts that went down recently. If he would have bought back bitcoins when it went down to $6,000 then he would have made another 50% by now. But seriously, it's quite hard to predict the price movement of bitcoins anyway.