I think you have some options depending on your general practices. We generally use the date of notice for our action taken date on denials. Was the counter a combined AAN? Did you send an AAN after rejecting the co-applicant? Depending on how you usually report your action taken and when you sent notices, you could use a few different dates. I wouldn't stray from your general practices.
1003.4(a)(8)(ii)- 2. Action taken date - applications denied and files closed for incompleteness. For applications, including requests for a preapproval, that are denied or for files closed for incompleteness, the financial institution reports either the date the action was taken or the date the notice was sent to the applicant.