While it's certainly uncomfortable to do so, it's important to let your friend know that you don't condone her bad behavior. Still, you shouldn't be condescending. Say something like, "I love you, but are you really sure that starting an affair is the right thing to do?" Or, "I care about you, which is why I don't want to see you get mixed up with drugs."
It's not always easy to get people off of a bad path, especially when they're determined to make their own mistakes. But if you truly care about your friend, it's important to stand up and try to guide her.
For more advice, contact a psychic. Our psychics can use their gifts to see if your friend is just going through a phase, or if you should intervene to help her before she causes a mess.