First, I'm sorry to hear of your loss. The best thing to do would be to contact the three credit agencies(TransUnion, Experian, and Equifax) and ask their customer service numbers what the procedure is for each to dispute the notation on your credit report. Likely the notation will not be on all three, but to do a thorough job all three agencies should be contacted. The agencies will likely need to see some sort of proof of your claim, such as a copy of a death certificate.
Be sure to follow up as well, because many people forget to do this. Oftentimes a credit agency will correct a problem but make another in its path. For example, a $14,000 debt to a collection firm might be corrected to a $0 debt to a collection firm without eliminating the notation. This might make some potential creditors hesitate to extend you credit because the $0 debt might appear like you defaulted and then settled for less. Make sure the entire notation disappears, not just the dollar amount.
NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. The author is licensed in Illinois only, and his answer is for educational purposes alone.
The reason that companies ask for a co-signer is to secure "another" person for the note.
A co-signer is responsible for the full debt of the contract. I would look at the finacne agreement that you signed and see what language is contained within regarding death of one of the signers.
you may want to hire an attorney because $14,000 is a large amount.