You would have to check with his company.
Some corporate cellular carriers plans DO NOT allow "Split Billing". Some do (AT&T does). HOWEVER, the company must agree to allowing Split Billing.
So need to talk to his company.
The alternative, which is much more expensive, is if the company iPhone has Dual SIM support (physical SIM and an eSIM), you he could get a full price private cellular plan for the eSIM and pair the Apple Watch to the private plan. This means he could ONLY answer private plan phone calls on his Apple Watch, as well as private plan SMS/MMS messages via the Apple Watch. iMessage and other services that are strictly Internet based, would not care.
NOTE: The GPS only Apple Watch can do Everything the Cellular model can do AS LONG AS your iPhone is nearby, where nearby is generally somewhere in the same moderately sized home. The Cellular model allows you to leave your iPhone at home while you go for a walk, run, bike ride.