If the debts are joint, then your partner and his ex-partner are both responsible for the lot, and your partner is also responsible for the debts in his name as well.
Your partner must now reassess his finances to decide whether he can actually afford to recommence contractual repayments to his creditors or look at bankruptcy, IVA or even a deparate DMP in his own right. I am afraid that there is little chance of him being able to take his former partner to a small claims court to recover her share of a joint and several debt.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk