How did she accumulate the debt in his name? Do you have any means of proving that she made the calls or spent the credit? If so, you can sue for the money in the Magistrates court or small claims tribunal in your state. These generally don't allow lawyers, so they are proceedings you conduct by yourself. They can be stressful though, so you will need to weigh up the cost versus the stress and the risk.
First thing to do though, is to write a letter of demand. You should specify the exact amount that is payable, where it should be paid to and the date by which it should be paid. You may also threaten legal action if the funds are not repaid within the designated time period. There are lots of sample debt demand letters online which should give you guidance.