1) Give up on on their legally acquired BTC, potentially worth millions of dollar.
2) Be honest on the form and pray that they don't get flagged.
3) Lie on the form.
There's basically no legal option for cashing out their BTC. A lot of early BTC adopters face this issue, as not everyone had the foresight to keep a paper trail for their 50$ "funny internet money" purchase at Starbucks.
Disclaimer: IANAL and this is not legal advice.