You both were still married on December 31. You need to file as MFS Married Filing Separate or Head of Household if you qualify. If she qualifies as Head of Household that's ok. She doesn't have to be Single to file as Head of Household.
But if she filed as Head of Household and Itemized Deductions you also have to Itemize even if it is less that the Standard Deduction. If she took the Standard Deduction you can do either way, Standard or Itemized Deductions.
Here's a FAQ for Head of Household
https://ttlc.intuit.com/community/family/help/do-i-qualify-for-head-of-household/00/25539
Can a married person file Head of Household?
https://ttlc.intuit.com/community/married/help/can-a-married-person-claim-head-of-household-filing-s...
