The IRS won't force the first spouse to go back and change their already-processed return. That's why the second spouse's return is the one that will be rejected, if doesn't comply with the first. If the second spouse wants to itemize after the first has filed "standard", his/her only recourse is to persuade the first spouse to file an amended return. The IRS is not going to arbitrate this matter; it's between the two spouses.
The quote in the OP's last comment merely states that if you CAN claim the standard deduction (because your spouse didn't itemize), you can only claim half the standard deduction amount that you'd get on a joint return.
Finally, it is definitely true that, if one spouse itemizes on an MFS return, the other spouse must itemize too - even if it is not to their benefit. This is one of the many disadvantages of the MFS filing status.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
