Nothing new here.
Verizon and Sprint have done this in the past.
It happened to me many moons ago. I had switched back to AT&T due to a job change and when I found their ding to my credit, I disputed it with the credit agency. (Also Trans Union) I told them that I hadn't had an active account with them for over 7 years, and the phone company disputed that fact. claiming I had a balance of something like 68 dollars on it.
Eventually it rolled off the record. About the only thing you can do other than pay the amount they claimed you owed. As to hurting my credit, it really didn't. The credit company had my dispute on record so that account was flagged.
But I wish this guy well and hopes he can win. It may end up being a Pyrrhic victory I'm afraid.