Check your state's w/c laws about the statute of limitations for finding errors. Many states have a 2-3 year statute, after which they can not collect on errors and overpayments. Write a protest immediately. $6,000 may be sufficient to hire an attorney to recover, if you can find one who will do a letter or two to shake 'em up. that is, unless they are within the recovery period. You could also see if there are nay provisions for canceling overpayments upon the death of a claimant.

I am so sorry for your loss. Hopefully this can be resolved somehow.