@Debst99 It should be clear from the many responses you received that there is no special 25% tax rate applicable to commission or any other type of supplemental wages, whether or not it is reported on a W-2 or a 1099-NEC.

The law on this is clear.  You just need to look at §1 and you will see that no special tax rate is available to any type of compensation.  Normal progressive tax rates will, therefore, apply.

Agree with @abctax55 that it would be unusual for a large corporation to make a fundamental mistake to not report the commission.  The only times when this is a problem is when offshore payroll is involved and that's when they need help from specialists.  I'd ask for a breakdown of the W-2 wages and reconcile that against the payslips - more likely than not, it's the employee who's mistaken.

In the event your client's employer did miss out the commission, your client should approach them to have them issue a W-2C.

I wouldn't expect a 1099-NEC - that would open up a can of worms with him being both an employee and independent contractor with the same entity - he could well be earning income from two distinct roles but there's not even a hint from the discussion thus far that that may indeed be the case.