Have your dad call, it was his account and account services will only speak to him.
When he calls, have him ask for account services. Have him give them an account number that was cancelled but not sent to collections. They can access all accounts through the one number and can best advise him from there.
I'm a little surprised though. Before anything was sent to collections, all of his other accounts should have first been turned off in order to prompt him to call in. Also, they should have been calling him at any/all numbers that are on file, not just sending mail that was being forwarded.
He can draw on his long term membership when he calls. The account can be called back, depending on the circumstances.
Best advise: be nice, be contrite, be honest. The people who work in that department are used to being yelled at all day, so a nice sounding person who honestly wants to work with them may just be given the edge that your dad needs.