Billed for 20 gigs of data

I was billed for 20 gigs of data on my wife’s mobile phone. She is disabled and never leaves the house so she is always on our WiFi  I have talked to two different people both of whom say that since it’s been billed there is nothing they can do. I have disputed the charge with my credit card company since I can’t talk to anyone at Xfinity that can help me