You respond to IRS with the same explanation you gave us:

"The brokerage 1099-B was for a joint account.  However, he and his wife divorced during the year.  So I reported half of the dividends, interest, and capital gains on each 1040 return.  Same with K-1s from a limited partnership.  I clearly marked (50%) on the Schedule B on each tax return.  "

I'd add the exwife's name and SSN in the explanation.