No. She should state her share of the income and outgoings only. There is also, I believe, a box on the form for partner's income.
The process that Magistrates use when setting a fine is to use the defendant's weekly net income and base the fine on a multiple of that. The other information, such as outgoings and savings, is usually for information only to gain a fuller picture of the defndant's circumstances and is not usually taken into consideration when setting the fine.
Incidentally, Magistrates' sentencing guidelines suggest a "starting point" for careless driving of a fine of between half a week and one and a half week's net income, depending on the seriousness of the offence. All fines are reduced by a third for an early guilty plea. She will also have to pay prosecution costs of about £85 and a Victim Surcharge of £15. I addition, assuming she is not disqualified (which is unlikely unless the offence is particularly serious) she will have between three and nine points inposed on her licence.
However, when setting the fine the Magistrates will consider only her earnings