Change from MFS to MFJ

I have a situation where a separated spouse (divorce pending) filed her 2021 tax return using the MFS filing status, and received her refund. I am now working on the husband's tax return. I was provided a copy of her tax return, and have determined that if the couple could agree to file jointly, they would net about $5,000 in additional refund. Since he has not filed yet, I believe to change from MFS to MFJ, her return would have to be amended to accomplish this by adding hubby's income and deductions. They could enter into some sort of binding agreement as to how to split the refund, but he would be entitled to the bulk of it. I presume that if her return is being amended, she would be the primary and the refund check would be mailed to her. Not sure if he is going to be comfortable with that. Can a refund check be mailed c/o a third party custodian to be sure it gets distributed per the agreement? Have any of you faced a similar situation, and how did you handle it? Thanks.

It's been a while since I dealt with this, but if I recall correctly, you don't amend the MFS return, you file a brand new joint return. Once that's filed, IRS will see that there's an MFS return and will mark that MFS return as a duplicate to free up the SSN for the MFJ return to be filed.

but if I recall correctly, you don't amend the MFS return, you file a brand new joint return.

That is correct. Don't forget to include any refunds received to either spouse or include payments from either spouse when the MFJ is filed.

The 1040X instructions seem to differ.
You are changing from a separate to a joint return. If you
and your spouse are changing from separate returns to a joint
return, follow these steps.
1. Enter in column A the amounts from your return as
originally filed or as previously adjusted (either by you or the
IRS).
2. To determine the amounts to enter in column B, combine
the amounts from your spouse’s return as originally filed (or as
previously adjusted) with any other changes you or your spouse
are making. If your spouse didn’t file an original return, include
your spouse’s income, deductions, credits, other taxes, etc., in
the amounts you enter in column B.
3. Read the instructions for column C to figure the amounts
to enter in that column.
4. Both of you must sign and date Form 1040-X.

Wondering out loud if it would be better to go ahead and amend since that would be the only way to e-file the return. I hate mailing returns in these days.

I think I would.

If I were representing the Wife (or soon to be former Wife) I'd advise to demand 1/2 of the tax savings $2,500 for the required Signature to endorse the join check. Beware cashing it with 1-signature is a crime.

Where the wife has filed separately and the husband hasn’t filed yet, and the spouses desire to file jointly, we’ve always filed Form 1040-X listing the wife’s SSN first and working from the wife’s separate return shown in Column A. This is what the Form 1040-X instructions say to do:

If you are changing from a separate to a joint return and your spouse didn’t file an original return, enter your name and SSN first.

I think I would.

I would too (I was the one who posted the incorrect, or perhaps hopelessly outdated, response). But the instructions seem pretty clear to me, and it's purely a question of administration, so I'd do what the IRS wants me to do.

Thanks everybody for taking the time to help me out!