Is there anyway i can get out of my ex husbands debt?

My partner and her husband have recently recieved their divorce absolute. However, he has in the last 2 years they have been separated taken their current account into over �3000 of debt. my partner did try closing the account at the time they split, however she was told the account was in the red and untill it was in the black she could not. having spoken to her ex he said he was trying to sort it and that it will be sorted soon. He then lost his job. my partner didnt hear anthing from the bank and so she umed that everything was ok. as it turned out he had told the bank my partners address was his mums address and therefor she was recieving no statement or correspondance. out of interest recently she called the bank about her mortgage. she informed them of her address. since which we have had letters from some third party debt collectors saying that if some sort of payment plan is not arranged soon we shall be getting a doorstep visit. we are both really worried bout it. his two children live with us, with no financial support from him. he has told the debt collectors that he is responsible for the debt (but i dont think that makes any difference) he has agreed a price plan of �5.00 per month. We then found out that he must increase this to �50.00 per month in the next 3 months, which we dont think he will do. nieve, yes! stupid even! but does anyone know how to get out of this mess without having to pay his debts for him with cash we dont have.