Credit Karma showed the name of the collection agency, so I called them up and got as many details as they’d give me (dates of the contract, email and address associated with the account).

Then I called AT&T to dispute the account, had to send in some proof of residency documents and swear that the account wasn’t opened by her.

Then I filed a dispute with Experian and TransUnion (for whatever reason it didn’t show up on Equifax).

It was a hassle, but about 3 weeks later it was removed from her credit and I’d gotten responses from the 2 bureaus and AT&T that the mark had been removed.