My daughter just turned 18 and she decided to withdrawl herself from out of school and run off with a 30 year old man. Please advice me on what legai grounds I have to stand on?

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

Unfortunately you have none. At 18 she is of legal age and can make her own decisions.

The only possible recourse you have would be if you could prove they had sex while she was under the age of consent.

You can't do nothing but hope that she does well and goes back to school. By law she is an adult and can do whatever she wishes as long as she doesn't do anything illegal.

I can see why your upset but she has to make her own mistakes in life and learn from them. Was dropping out of school wrong, yes but there is nothing you can do. One day she is going wake up and smel the coffee.

You can wish her well, tell her to call when she wants and tell her you hope she will be happy.

By not burning the bridges behind her, you allow her to be able to come back if she fails.

Legally she is a adult and can do any stupid thing she wants to do.

This could be an act of defiance and like the person said before don't burn your bridges. I was 18 not long ago and can remember doing stupid things to get a certain reaction. Don't get upset act like its not a big deal no matter how big a deal it really is. Being a parent is hard there is no manual. All you can do is support her and pray for her.