You and the other person currently both have a responsibility to repay the debt. (i.e. the responsibility is 'joint and several' which means that either, or both, of you can be pursued for full repayment of the debt). If you enter into an IVA the other person's responsibility for settling the debt remains unchanged. (i.e. they can still be pursued for full payment of the debt).
This might be relevant to your situation:
http://www.freeivaadvice.co.uk/Joint_debts.htm
Chris