Salary Deferrals are subject to FICA taxes.
Employer contributions from corporations are not subject to FICA taxes
Your CPA sounds correct in his analysis given your stated facts and desire to contribute exactly $25K.
In fact if you only want to contribute 25K you don't need any salary deferral, you can simply make a 25K PS contribution since you have 100K in w-2 wages.
If you want to contribute more than 25K, then you could make an additional salary deferral of up to $18K ($24K if you are age 50 or older), though that needs to run through payroll and time for that is getting a bit short.
edit - though in the long run it doesn't matter if your gross w-2 wages are $100K you will pay the same FICA taxes if your salary deferral is $0 or $18,000.
The difference will be whether you have more pass through income reported to you and a lower taxable wage on your W-2 or a higher less pass through income and higher taxable wage in your W-2.
Now if you have to raise your income to $118,000 to make an $18,000 deferral that would change my answer.