In the 'good old days' when a visit to the sharp end was possible for SLF, it was usually not allowed during take-off or landing. Even if the passenger is some billionaire banker, you never know if he might go bonkers and try to crash the aircraft during landing when both pilots are busy.
IMHO the Captain was an idiot, especially to make his remark upon landing. I hope Mr Diamond will find him job when he gets fired.