The surviving, remarried spouse has been repaying only half the credit, right?  (I thought these things went on for only 10 years, but apparently it was 15.)

Person who claimed the credit dies. 

If a person who claimed the credit dies, repayment of the remaining balance of the credit isn't required unless the credit was claimed on a joint return. If the credit was claimed on a joint return, then the surviving spouse is required to continue repaying his or her half of the credit (regardless of whether he or she was the purchaser) if none of the other exceptions apply.