How much tax was saved by filing separately after making joint ES payments?  I hope it was a lot.  Has the husband's return been processed yet?  I would wait a couple weeks and see if he gets a refund, then mail it back marked "Void" with a letter asking that it be applied to the wife's account. 

But then, is the wife receiving a bill for the missing payment?  Or just a reduced refund or credit-elect?  

Are the spouses still on good terms, or were the separate returns the result of a separation?