Elderly mother put on new contract without her consent - is this legal?

My 85 year old mum rang Virgin/150 as her TV service wasn't working.   After arranging an engineer visit, the customer service agent told her she was out of contract and tried to get her to sign up to a new one.  She repeatedly refused and told him she needed to see what the offer was on paper or email before agreeing to anything.  He started a new contract anyway.

She only realised when she checked her email a few hours later.  She spent another hour on the phone trying to get this new contract cancelled.  Repeatedly telling them she wanted to cancel as she had not agreed to a new contract they just kept putting her on hold and then trying to get her to keep the new contract.  At the end she was in tears, only then did they cancel it and revert to her old contract.

I am absolutely appalled. Is this anyway to treat customers, especially elderly people?

How is this legal?  

Is there any point in complaining? What recourse do we have?