Hi caz
From the HMRC website -
When we work out your tax credit payments, we look at whether you're single or living with a partner. If you split up with a partner or a new partner moves in, your claim legally ends and you need to set up a new one. If you are claiming tax credits and you split up with a partner or a new partner moves in, you must let us know within one month.
If you don't tell us about changes like this we might pay you too much. We'll usually ask you to pay back any overpayments and you may be charged a penalty.
If you're still able to get tax credits you'll need to make a new claim. If a new partner has moved in, you'll need to make a new joint claim together. You'll need to fill in a claim form. You can get this by calling the Tax Credit Helpline. Once we get your form we'll send you a new award notice telling you what tax credits you'll get from now on. Please check that your details are correct on the new award notice.
If you don't qualify for tax credits any more, your claim will end on the day you split up with your ex-partner or your new partner moved in. If we pay you any more tax credits after that date it'll count as an overpayment that you'll probably have to pay back.
Basically if she's now living as a couple but claiming as a single person, she needs to let them know. They do have a tendency to find these things out anyway sooner or later, so I'd say it's more a question of when, rather than if they find out.
If she's got access to the internet she can always use the benefit calculator on their site
HM Revenue & Customs: Tax credits calculator to see what her new amount would be (if she's still entitled to it that is). If she then splits up with her partner she can still just change it back to a single person claim, so it's no biggie there either.
Btw, if the link above doesn't work she can just type 'benefits calculator' into Google and there'll be tons of results available that she can use.
Hope this helps!
Lexis200
