Grr, getting really incensed here on your behalf - I agree with clam that there may be an element of manipulation involved but stay strong!
Even if legal action is threatened or carried out against your mother it is still not your fault! She has plenty of options to sort out this visa bill- (firstly stop spending! try and arrange money from her working husband, get a job, remortgage the house, CAB, payment plans etc - none of these may be easy or palatable options for her but SHE needs to take responsibility for the debt, not you!
Most early demands for payment can be sorted out by calling the company, being honest and trying to work out a means of repayment. But the longer she leaves it the worse it will get, and the more difficult negociations to pay might be. She can still sort this, you are not her last resort and you have to keep telling her this like a broken record. If you can, offer her help to sort it out herself eg calls, letters etc but please dont accept responsibility for her debts!
(And just to help you stay adamant imagine how even more sick you would feel if it was a demand for payment of a loan in your name, coming to your home, because she and her 'husband' dont keep to their side of this dubious arrangement they are suggesting)