My friend asked me to post this on her behalf. She is applying for JA but has a mortgage on a house (which is negative equity). Will this 'asset' affect the likelihood of her claim being successful?
Since she is in negative equity can I take it from the above sentence that the capital value is effectively nil and thus this won't be counted against her?
Yes, that is the case - the rent she receives for the house is not assessed as income, it's the property which is considered the asset (which in this case has a nil/negative value).