Hey, thanks for the question. I know that seeing one of your best friends get into drugs is a hard thing, but I'm glad you care enough to try and help her. I noticed you said you've never done drugs and you don't plan on doing it, and that's GREAT!
As for your friend, life does not have to be stressful or worrisome for her to use drugs, some people are just curious and others feel pressured to try drugs. No matter what the reason may be, the result of trying drugs can be damaging.
You could share with your friend the effects Marijuana has on the brain and body. THC is a chemical compound that produces the physcoactive effect. When THC is in the brain it connects to a site called cannabinoid receptors. These receptors are often found in the parts of the brain that control memory, thought, pleasure, concentration and etc. You can go on http://www.justthinktwice.com; http://www.justice.gov/dea/concern/concern.htm for more information on Marijuana.
The best thing you can do is continue to be a positive influence and be there for your friend. One last thing, if she tries to get you to smoke marijuana I hope your able to make the right choice to continue to be drug free because your health and well-being is so important and helps you to help others. Good luck, I hope things work out for the best! (: