I will be installing a standby generator located about 50 ft. from the
grounding rod for my house. If I ground the generator only in main panel,
then there is a possibility that the generator frame will be at a slightly
different potential than the adjacent earth on which someone touching the
generator may be standing. If I install a ground rod at the generator
location, then I will be introducing a ground loop with its associated
problems.

How, according to the NEC, should the generator be grounded? Please
provide a code reference (or quote) if you can.

Many thanks for any help.