The W already filed her 2023 return?  Someone must really hate procrastination!

Did the Block file HoH because H & W did not live together during the last 6 months of 2023 despite the marriage?

If they should have filed MFJ, they can amend their returns to elect joint filing.

Not that it matters in your client's case but there is a position that changing from HoH to MFJ may not even be time barred by §6013(b), based on prior court rulings because the Congress, in enacting this code section, narrowly scoped that to be a change of election from MFS (and not from any other filing statuses) to MFJ.  It was ruled that the IRS' Rev. Rul. is unreasonable in treating all returns filed by a spouse with status other than MFS as being separate returns for purposes of §6013(b).