she may have been entitled to file as HOH
a married taxpayer who is a US citizen or resident can file as HOH IRC sec 7703(b) if all the following tests are met
1) she files her own return
2) she paid more than 1/2 the cost of keeping up their home for the year
3) she lived apart from you for the last 6 months of 2019
4) her home was the main home for more than half the year of her child, stepchild, adopted child or foster child. the child must be a dependent or would be a dependent except that she released the exemption to you - the noncustodial parent
then all you can do is file as single
if there's a failed test all you can do is file as married filing separate - you can not force her to file a joint return. note if she itemized so must you or if she took standard so must you.