This is such a tricky subject to broach when you are talking about new relationships, previous debts etc.
Maybe the best way to explain it is that ALL income into the house should be declared to show transparency, and whilst nobody would expect an innocent partner to pay for previous mistakes, it is equally unfair to ask businesses that have lent money, in good faith, to potentially write off debt because a new partner is expecting the bills to be shared in a disproportionate manner.
For example, £3k income in the house is made up of £2k her and £1k him. The bills are £1k per month so they agree to pay £500 per month each. In reality, and, crucially in the eyes of the creditors, he is subsidising her to the detriment of the creditors to whom he owes money.
Personally, whilst seeing both sides, I would not be happy, if I were owed money, to be told that I have to take less in the circumstances described. At the end of the day, IVA proposals are subject to the approval of the creditors, so in the end it doesn't mean that she has to pay his debts, it simply means that he has to cut his cloth accordingly if that is the product that he wishes to use to service his debts. If he, or she, is not happy with that then they have to decide to go down another route, whatever that may be.
Hope that this makes sense.