Yes.

18 U.S.C. § 793

18 U.S.C. § 951

It isn't a legal matter for them. The Board cares just about the value of NVIDIA on the Stock market, and whether any industrial secrets have been leaked.

Yes there are laws against participating in espionage.

Although a board of directors/shareholders doesn’t necessarily have to care about it being illegal. It just looks really, really bad that the CEO of the company they have invested in and help run is having an affair with what turned out to be an actual spy.

When the company looks bad, the company loses money. Haung’s entire job is to make the company more money. They can try to force him out just on the merits that he has damaged the company stock value.

Much of the time, it's illegal if the person being manipulated/used/complicit knows, so the FBI will often tip them of an intelligence asset targeting them so if they keep it up, or spill the beans when told not to, they can get charged.

Maybe there are other laws too but that one came up with the US president's first term investigating all the russian links to the campaign back before we officially became south america where nobody gives a shit anymore.