Will receiving child maintenance affect tax credits?

My step daughter is currently living with my husband and I. She is 39 and has two girls aged 2 and 4. She is working full time. Her husband is currently in Scotland. They separated last autumn. My step daughter tells me that if she seeks maintenance from the father she will be deducted by that amount from her benefit which I think is some form of tax credits. Is this correct. If it is and the father doesn’t contribute then the tax payer is paying instead. She says that currently she gets 85 percent towards child care which will be affected if she seeks maintenance from the father.