It is not correct that the taxpayer is paying the debts. If the person is in an IVA that means they are making payments to their creditors for 5 years & - at the end of the 5 years - any part of their debt which has not been paid is written off by the creditor. It is the creditor who loses, not the taxpayer.
I do not understand what they mean by not declaring something. They must have declared all their debts because any they didn't declare would not go into the IVA & they would have to go on paying them. Do they mean they have failed to declare all their income? If so, the payments they are making on the debts would be lower than they should be.
If you want to grass them up you need to find out who set up the IVA for them - it has to be a firm including Insolvency Practitioners but there are many such firms & you can probably only find out from the person herself. Once you know that you need to contact the firm, but unless you have something fairly definite to tell them they may well not get anywhere because the woman will deny failing to disclose anything.