1. She needs to get some proof that the money is owed. The company will probably not help much, but she can ask for full details of their dealings with her under the Data Protection Act - I think it costs �10. Google it to find info. She could also ask her ex (if contactable) whether what they are saying is true.
2. If she does owe the money then she needs some free debt advice - go to local CAB or any other FREE money advice service. Normally this sort of thing (bankruptcy) is threatened without any serious intention to carry out the threat, but she cannot assume that will be the case. In most cases, if the creditor is at all reasonable a monthly payment offer based on a Financial Statement of her income & essential expenditure - i.e. the amount she can reasonably afford - should be accepted.
3. If the worst comes to the worst and they do go to the stage of starting bankruptcy proceedings by issuing a Statutory Demand then if she can somehow borrow some money to get the debt below �750 they cannot make her bankrupt.