Technically speaking, if you died and she continued to do that, she would be committing an offence under UK law whether it's a joint account or not.
Now it's none of my business and I'm not trying to lecture you or be "holier than thou".
BUT, if she got found out she could be prosecuted in the UK (I think it's classed as fraud) and told to pay the money back. The DWP could lock the bank account down. Of course if she never goes to the UK and the bank account was empty, there's little they could do ????
Having said that, a late friend who lived in Thailand for 30 years was sent a questionnaire by the DWP when he was in in his late seventies. The questionnaire basically seemed to be asking him to confirm he was still living at the address they had for him. I have no idea why, or how many pensioners get such a letter, just mentioning it in the light of what you said.