I'm sure he can think of something. This part is best left to your imagination. The drug guys and organized crime have been doing it for years. Or you could mix it up and use your crypto to buy PMs (like gold or silver) from a reputable site then sell those to a PM dealer local to you. (Big transactions will get recorded.)
Or, you know, you could always just say that you got your bitcoin from Mt. Gox. Or from Cryptsy. Or from [
insert any other exchange that disappeared here]. How would they know otherwise?
But they don't work like that, the just put a hold on your money and say prove what you say is true.
You effectively need to prove your innocence.
I have already pulled together a bunch of historic records from all the places I bought bitcoins from back in the day.
Plus having some old private keys of where your coin used to be will help, as you can sign a message saying that this address belongs to me. So you can show a trail.
(I think for most people this is only important for property purchases)