Smoking weed isn't necessarily a bad thing. But anyone who does, except for medical reasons, is breaking the law. Good people don't break the law. I'm pretty sure they didn't make "one mistake." Giving her the benefit of the doubt that this was her first time, pretty sure it wasn't his first time. She did it because she allowed him to influence her. That's when you see the lying that comes with illegal behavior. Good people don't lie to try to cover up or justify doing something illegal.
Yes, a lot of people smoke weed. But that doesn't make it right or okay for your friend to. My mother used to ask me "If all your friends jumped off a cliff would you jump too?"
No, you're not right. Your friends parents make the rules in their home. They don't want their daughter or anyone else doing anything illegal in their home. As a parent, I would never let him back in my home. He is no longer someone to be trusted. And I would be worried about a daughter that was week enough to be influenced by him.