It's his. Can't fob it off on the ex.

Better get it paid in with the return so she can pay her half 😁

Brings to mind an old song by Jerry Reed "She Got the Gold Mine, I Got the Shaft".  One of the lines is "they split it right down the middle, and then they gave her the better half"

If they’re MFJ this year they’re both your clients. The retirement monies are likely marital assets and how to handle the tax consequence should be done by agreement... or let the lawyers fight it out.

if you don’t have agreement of both parties, your setting yourself up for future aggravation 
I generally take the position that the objective is for the IRS to (legally) get the least amount of tax and whack up the burden fairly.