The expense is at least prima facie legitimate. On paper you’re just paying a fee to a contractor in exchange for a NDA protecting the company from liability. If you cover things up well enough, it won’t raise suspicions and nobody is going to go out of their way to investigate their boss.

At a certain point this sort of thing does become unsustainable, but most execs stop short of that point. Also, this sort of thing becomes illegal at some point as well; you can learn about cases where people didn’t get away with it, and then maybe extrapolate from there to see how other people might get away with it.