https://www.law.com/therecorder/2019/06/11/employee-nonsolic...
Edit: above was paywalled, and I didn't realize I was trial-viewing it. Think this is free for all:
https://www.lexology.com/library/detail.aspx?g=5993e280-c03b...
They were only questionably enforceable before too, since they still amounted to a contract restriction limiting someone's employment opportunities (in this case, the person being recruited).
While CA does codify non-competes (and now non-solicits) are void, general interpretation was that any restrictive contract clause wasn't going to fly in court except under very specific circumstances. CA takes a very dim view of restricting job mobility.
Ethically, totally slack if he recruited while still employed, of course, and if he literally did it at work I suspect he'll run into a "using company resources (time, building, hipchat channel, etc)" issue, which is one of the things that severely weakens a defense that whatever you do yourself, you still own.
No idea how or if that applies at a high-level action like starting a company, but if he had any technical contribution I could definitely see it coming up.