My husband works for a company in NJ that has about 60 employees, all of which are hourly, including Managers.
Last week he found out that the HR Manager is salaried. If this is the case, shouldn't all Managers be salaried? If so, can you direct me to the regulation?

No, you are proceeding from a false assumption.

There is no law that any employee EVER be made salaried. Nor does it necessarily follow that if one manager qualifies as exempt, all other must also.

It is not the title, but the job duties, that determine whether or not an employee qualifies to be exempt (salaried is only a pay method).

EVEN IF an employee qualifies as exempt, it is still legal to pay them as non-exempt. Since the reverse is NOT true, it is much safer, if there is any question whatsoever about their exempt status, to pay them as non-exempt.

Thank you.