I have a friend who want to stop her daughter's father from claiming her on his taxes?

Nothing if he has custody of any sort. Even if it is partial or even if he sees her 1 time a year, or sadly even if he pays child support and never sees her. Monetary or items period given to her, he has a right to claim her. There is nothing you can do short of a court order which will then legally bind them to not be able to claim, if then that happens the father will have to repay back the govt as well as fines and possibly jail time due to tax fraud IRS doesn't take kindly to Tax issues when it comes to money coming out of their pocket illegally. I have been through this personally with my own father, and my mother, and the upheaval of this. My father did the same thing, then my mother couldn't claim at all.... he was legally allowed to do it. IRS told my mother to get a court order to stop it. You have to have a good reason and usually it is the reason of whoever has the most custody usually wins, meaning who ever pays more for the childs upkeep has the right to claim the child, this is the easiest when who shelters the child more, and 2ndly who feeds them more. Just prove that then you are good to go.