if it doesn't say ' no night flying' on his FAA certificate he is good to do so.
Not on a US-registered aircraft. The 2012 opinion to Krausz furnishes that as an example of an endorsement (a clearly defined privilege limitation) which, if made in a foreign licence, constitutes a limitation for the purpose of 14 CFR 61.75(e)(3). In any event intentionally operating as pilot-in-command at night without receiving night flight training is careless and reckless.
Validation (sic) is a one time deal ...
A valid letter used to be required before attempting a practical test when Notice 8700.15 was effective shortly after 911. Whatever the sequence, additional ratings cannot be included in a restricted foreign-based certificate without there being a valid verification of authenticity letter. Refer to 5-603 in Order 8900.1.