my first concern was they may try and dump them today. on smaller exchanges or trade into alts, then sell.
or they could just secure in multiple wallets and leave for years. then sell when everyone has 'forgot'
maybe a dump is on way today or they may sell on localbitcoins slowly, who knows
They have to sell it slowly. There's no other way. No one will buy such amount with cash and if they start selling large amounts on any fiat/BTC exchange - that would immediately raise suspicion.
Even if they successfully convert to fiat, there's still a risk of a taxman asking where did you get the ~$5m from, or bank freezing your account etc.
I reckon all the process will take months if not years.
Or he could just live off the Bitcoin? I mean think about it, he can buy gold, silver, lambos, booze, clothes and pretty much whatever he wants. He just needs to not let the tax man see him living too lavishly.