Hardware keyloggers, microphones, spycams, etc.
The precautions you describe does not shield against any of those, and I'm not even being creative.
> Hardware keyloggers, microphones, spycams
You're right, haven't thought about those.
in that case the rootcause is letting other people into your house, not cheating
All of it together. He compromised the company by having an affair with a Chinese spy. The cheating is relevant.
If that’s all it takes then he should never have any friends either. They could be compromised too.