If she is an adult now, you cannot sue him; only she could. A mentally competent adult must bring her own lawsuits, and no one can assert claims for her. It does not matter if the harm occured when she was a child; it is her claim, not yours. While you could have brought it for her while she was a minor, since parents have legal guardianship over their minor children, once she turned 18, your authority over her and legal control lapsed. So she could bring a legal action, but you can't do so for her.You could report this to the police and the authorities may, depending on exactly how long ago this happened and the circumstances, bring legal charges, but they are unlikely to do so if your daughter--the victim and key witness-doesn't want them to; they will likely respect her wishes in this regard.