S Corp filed return and wants to file an amended return

Received this question from a CPA:

"An S Corporation filed their tax return by the 3/16/2020 deadline, and now has decided that they want to amend their tax return. The amendment would make an accrual and contribution to their retirement plan. I know they were supposed to put their tax return on extension for making this contribution, but I’m wondering just how bad it would be if they did this anyway? I think if the IRS did an audit and the payment wasn’t paid by the deadline including extensions, then they can disallow the deduction, so in that sense they will be rolling the dice with the IRS, but are there other issues, perhaps fiduciary liability issues, that I should make the client aware of?"

The taxpayer (corporation) can do whatever they want.  However, is the CPA going to sign the return as the paid preparer, knowingly taking a deduction that the corporation is not entitled to?   There isn't really any arguable position here, the deadline is passed for 2019 deductibility for this corp. 

How does this comport with Circular 230 and/or  the CPAs' state board of accountancy requirements?  

Even if a practitioner is not a member of the American Institute of Certified Public Accountants, many States’ laws impose the AICPA’s standards or similar standards.

AICPA Statement on Standards for Tax Services No. 1, Tax Return Positions:

¶ 5(a)      A [CPA] should not recommend a tax return position or prepare or sign a tax return taking a position unless the [CPA] has a good-faith belief that the position has at least a realistic possibility of being sustained administratively or judicially on its merits if challenged.

¶ 5(b)     Notwithstanding paragraph 5(a), a [CPA] may recommend a tax return position if the [CPA] (i) concludes that there is a reasonable basis for the position and (ii) advises the taxpayer to appropriately disclose that position.  Notwithstanding paragraph 5(a), a [CPA] may prepare or sign a tax return that reflects a position if (i) the [CPA] concludes there is a reasonable basis for the position and (ii) the position is appropriately disclosed.

Few taxpayers are eager to file an audit-me Form 8275 (or 8275-R), so the standard often is whether the practitioner believes the tax-return position has a realistic possibility.

I express no view about the underlying tax-law question.