It depends how long she is staying and her circumstances.
Ring and ask the council
Second Adult Rebate
You may get Second Adult Rebate if the person you share your home with is:
not your partner or civil partner
aged 18 or over
not paying you rent
not paying Council Tax themselves
on a low income
You may be able to get Second Adult Rebate even if you don't receive Council Tax Benefit. If you're either:
not entitled to Council Tax Benefit
only entitled to benefit which would cover 25 per cent or less of your Council Tax
Your local authority should automatically check if you're entitled to a Second Adult Rebate.
You should also write to your council if you are no longer entitled to a discount.
Visit the Council Tax pages of your council's website
http://www.direct.gov...ouncilTax/DG_10037422