Assessment for an IVA is generally based on how much is owed and how much you can afford to pay back each month. If state pension or other benefits are the only income, then it is unlikely that the person would have sufficient income to be able to make meaningful payments.
However, IVAs are "sold" by a variety of companies, some of which have been known to put through IVA proposals which should really never have been made.
In most cases, unless the debtor has property to protect or is in a job which they would lose if bankrupt, bankruptcy is a quicker & better solution than an IVA.