1) No harm in declining this engagement and letting them find someone with more experience.  In my experience these things can get messy pretty quickly if there are a lot of transactions.  I have no problem leaving these messes to the "next gen" accountants (are there any next gen accountants?  I think we're a profession that's slowly dying of old age and stress.)

2) When he's paid, the crypto has a value in USD based on the date/time of the transaction.  That's the Sch C income and becomes the tax basis in the digital asset.

3) When he sells it, converts it, spends it, etc. that will trigger a capital gain/loss.  It's not double taxed because it's not the gross amount that's subject to taxation at that point, just the difference between amount realized and cost basis (similar to a stock sale--except they don't take fractional shares of MSFT in exchange for coffee at SBUX!)

4) There are all sorts of rules for mining and forking and staking which may or may not apply depending on what else is happening.

Rick