Hello and Thank You for using Just Answer. I am sorry to hear about the problems that you have experienced with your daughter. You did receive legal advice so you are aware of that route. I am a Parenting Expert so this is my take on the situation since you have requested parenting advice.
1. Explain what you know about the entire situation with her dad. The fireworks will flare but you both must be on the same page with this issue. It is better that he be told than to find out another way and then later finding out that you kept this a secret from him.
2. You and your daughter's dad (husband?) need to sit down and discuss all that you know about the situation with your daughter. You must be open and honest with her. Discuss how he can be categorized as a sex offender. She may think it is "cool" to be hanging out with an "older" guy but explain to her how she is being used probably for sex. He most likely showers her with gifts and money to keep her around. Let her have time to explain her side of the situation as well. Then tell her that she needs to stop seeing him. She will have a royal fit but then it is time for tough love. If you find out that she is still seeing him, punish her. Limit her out of the house activities. Limit her computer time and cell phone usage. Make her feel as though she is in jail.
3. Then...this is the hardest step if the relationship does not cease. You and your daughter's dad need to contact the boyfriend and/or his parents. He and his parents may not even know the true age of your daughter. Tell him/them that you do not approve of the relationship and even bring up the entire legal information if desired. Explain that if she was 18, there would be no problem. But as her parents who are her legal guardians, your desire is for the relationship to end. You may want to make a phone call instead of an actual face to face visit. You never know about people these days and if they may become violent.
4. Then there is the legal route which you may want to pursue anyway.
5. You can also call your local child abuse department. They are available 24 hours and they can even provide more advice if you do not want to contact the authorities yourself. They will begin the investigation.
I really hope this helps you. I wish you much luck with getting your daughter back on the right track. If you need more advice, please feel free to let me know. Have a great rest of the week.