There is value for an 18 year old to work for a bit before trying to work a start up. It will help him meet people, pay rent, save some money, maybe learn some new tools, and something about management and business.
You missed the part where he has a profitable company already. He does more management and business at his own company than as a low level engineer at a megacorp.
A better answer is that his startup is done and generating cash flow with part time work, and doesn't really have a future growing to really challenging DocuSign.
A better answer is that his startup is done and generating cash flow with part time work, and doesn't really have a future growing to really challenging DocuSign.