Just to provide some additional context.  No deduction of SE-tax is allowed on Sch C because it is already allowed somewhere else on the return (well, at least a part of it - more on that later).  Allowing a deduction on Sch C would be double dipping.

First, you need to remember that SECA (under §§ 1401 and 1402) is the self-employed's equivalence of FICA (under §§ 3101, 3111, and 3121).  For FICA, half of that is recovered from employees, which means employers can only deduct their half as a business expense.  The same goes for self-employed individuals - they are only allowed to deduct half of the SE-tax, which represents the employer's portion of OASDI and Medicare.

Here's where it gets even better.  SE-tax is assessed only on net earnings from self-employment, which excludes under §1402(a)(12) what essentially is the employer's portion of SECA.  That is why SE-tax is assessed only on 92.35% (100% - 6.2% - 1.45%) of the net earnings from self-employment.  As a side note, this is not perfect math - if Congress wanted to do it right, they would have recognized that this is a circular calculation but that could confound the general public.

But that's not all... If you call in the next 10 minutes, we will give you another discount.  (Monotone: Operators are standing by.  So call now.)  Oops, I'm sorry - that's for informercials.  In any case, the other discount is the deduction I referred to earlier.  §164(f) allows 1/2 of the SE-tax as deduction that is attributable to a trade or business, which is deductible for AGI under §62(a)(1).

At the end of the day, your client will get a deduction for SE-tax, just not on Sch C but on Sch 1.