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.
avoiding banks is not a crime.
[0] https://en.wikipedia.org/wiki/Financial_Action_Task_Force
You got lucky and/or maybe forgot that you supplied that information when you signed up. Sometimes it can just take the form of asking for occupation and income. It's possible that they didn't ask further proof if they determined that your transactions were not unusual given your declared occupation and income.
[0] https://coinbase.com/legal/user_agreement/united_states "Verifying your identity by submitting the following information: [...] Source of Funds"