I started smoking pot when I was 13. I smoked for about 8 years, so I feel that I can confidently speak about MJ use...
That said, I think there are some very important things that your dd needs to know about, and they may not be the ones you think. If you know your child is smoking MJ, and you do not condone usage, make sure your child knows:
1-that you do not approve
2-that they can always talk to you about it openly without being shamed or judged harshly
3-that they know their legal rights.
That last one may not be a discussion you wish to have with your child, but it is very important for people who do smoke pot to be aware of their 4th Ammendment rights - or, the protection against illegal searches and seizures.
Like it or not, your child is smoking MJ and it is illegal. IF she chooses to keep smoking, she will probably be involved in some sort of police situation eventually - esp. if she is 13 and smoking with other kids. Kids are much more likely to smoke pot in places like cars and parks where they are easily apprehendable by police/rangers, because they usually don't have a house they can smoke in. (Where the risk of police intervention is much less.)
When I was younger, and still today, I had many friends who ended up in legal trouble because they did not know their civil rights. If you feel your child will continue somking, I recommend making sure she understands:
1-She can deny a police officer a bodily search, including personal belongings. The officer will then have to obtain a warrant before legally searching.
2-She can be easily bullied into thinking they "have to" let a police officer look in their backpack, pockets, etc...but they do not. Police officers will frequently patronize teenagers by saying things like, "You don't know your rights", "I AM allowed to search you", etc.
3-Smoking pot in a parked car or a public park is just about the worst possible place to do it. Smoking in public=trouble.
4-The 4th Ammendment does not apply to public schools for minors - meaning they can have their belongings searched any time without permission or a warrant while at school.
I know it may send a mixed message to your dd to say "Don't do it, but this is how to do it better," but if you feel she will continue regardless, you owe it to her to educate her to use wisely. That is very very important.