Thanks for your question
As we cannot advise a specific calculation for tax credits. I have added a link for your girlfriend to use, which is the tax credit calculator, which will allow her to put both of your details into, to establish the amount of tax credits, your move in, would affect her claim by. This allows a confidentiality for your girlfriend with her own personal details and earnings. I can advise though that her tax credits will be affected by you moving in, as her claim changes from a single claim to a joint with a higher income, but this will allow you to establish the true cost.
Do feel free to ask any follow up questions, but if you could be kind enough to rate the level of service I have provided, it would be appreciated.
I hope you found the calculator helpful, can I be of any further assistance?