Everyone is getting tied up in tax rules, and suggesting talking to an accountant. What about the PPL privileges, though?
Your friend must, according to the privileges of his PPL, pay his "fair share" of the cost of the flight. That means that if he's flying solo, he must pay for the entire flight, regardless of whether it's for business or pleasure.
That aside, there is nothing illegal about flying for business on a PPL, so long as your friend isn't
required to fly as part of his job (which shouldn't be difficult to fix, since he owns the business!) Wish I could have a job where I could fly for business!
FFF
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