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@jofijohnjoseph wrote:

Unlike claiming a child, there is no right answer for the IRS here. They don't care if a MFS couple claims the SD or itemized, they just want the same treatment for both.

Unless you can tell me otherwise, it does appear that whoever files first establishes the default treatment.  And if the second spouse chooses differently, both spouses are in theoretical trouble with the IRS.

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You have a fundamental misunderstanding of the rules.  There is no rule that both spouses must choose the same deduction method.  Have a look at IRC 63(c)(6):

In the case of—
(A)a married individual filing a separate return where either spouse itemizes deductions,
the standard deduction shall be zero.

So, as Bob alluded to, if your client is going to itemize then it's not relevant what the spouse does.  If your client is going to take the standard deduction, then your quandary is whether that's valued at $0 or whatever the MFS inflation adjusted amount is for the tax year (and if valued at $0 you might as well itemize.)

Rick