Im going to state my opinion and probably get flamed for it but I really dont care. I was that hard headed teen that did not care or listen to anyone. No matter how much you tell her your reasoning for your rules, reguardless of how right or wrong they may be, she will not listen. I honestly feel im right. She has already stated it and has went behind your back and ran away. She is past the point of just talking to. She will have to learn from her mistakes after she makes them to realize that you were in fact right and doing this for the right reasons.
Like HiSalenity said, she is socially behind her peers. This puts SO much stress on her to see her friends going out with guys and talking about dating while she sits back and says shes not allowed because her parents dont appove. I am not saying what your doing is wrong, Im just telling you how it is standing in her shoes.
There is not much difference in 15 and 18. Legally, yes. Realistically no. And to label a young man a sex offender when in reality he didnt "offend" anyone except you and your wife is IMO ridiculous. I dont feel like its a parents choice to choose to file charges on a guy when the daughter wanted to have sex just as much as the guy. That is ignorant. Not that they did, im speaking hypothetically.
It seems threatening some sort of action is only making her rebel more. Maybe try asking her how she feels about the rules yall as parents have set, ask her what she would change if she could. Maybe yall can find some common ground.
I would DEFINITELY talk with the guy. Let him know he needs to respect her, and yall as parents and your wishes. I would remind him that any sexual relations are illegal due to the age difference. Maybe the thought in his head will deter him? IDK.
I say all of this because I was the kid that didnt listen to anyone and made alot of mistakes. No matter how many times my mother talked to me in many different ways, I still had to learn on my own. Luckily I didnt do anything life changing, but I did cause alot of hard times on my family. I respect them and love them for trying everyday.
Listen to your daughter, really listen. I hope she comes around and realizes what shes doing is wrong. I hope the best for your family. Good luck brother.