An interesting news article covers most of what you need to know on the federal level.  One IRS executive noted, 

"This kind of information is also available on the IRS website, the official said, though he noted for now people need to search “marijuana,” rather than “cannabis,” to find what they need. That’s changing, Harris said, “because I really feel like a lot of the folks that are in the industry prefer to use the word ‘cannabis’ instead of ‘marijuana.'”

https://www.marijuanamoment.net/irs-official-gives-marijuana-businesses-advice-on-tax-compliance/

The two most notable things about this business are, to me:  (1) ordinary and necessary business deductions are not allowed, but you still get cost of goods sold; and (2) banks won't let you open an account, so it's all cash.  (Many "dispensaries" have ATM's inside.) 

California keeps changing its rules.  They just repealed a 15% excise tax on cannabis.  That and other information here:

https://www.cdtfa.ca.gov/industry/cannabis.htm  

If your client hasn't started selling anything yet, is he just testing the products himself for now?