I'm slightly confused.
Normally, you can't withdraw from a 401(k) while you are still employed, except for a hardship withdrawal. If he made a hardship withdrawal but still works for the sponsoring employer, then you will be subject to regular income tax plus the penalty because the withdrawal was not made after he was separated from service.
If he also separated from service with his plan sponsor so this was a regular withdrawal, and he separated before the year he turned 55, then it is also a non-qualifying withdrawal and subject to the penalty.
But if he separated from service from the sponsor of this account in or after the year he turned 55, then it is not subject to the penalty, because he would qualify under the separation rule.
