Because it was his birthday and she had company over so she didn't wanna start a scene. She didn't necessarily seem okay with it to me. She just let it slide.

"extremely dangerous"? Gimme a break, you need to relax and not be so alarmist.
He was 15, and he obviously wasn't "drunk" he came in quite normal, just a bit tired. Did you see him stumbling? No, you didn't.
Cannabis is one of God's plants. It grows naturally in the ground, nothing artificial and is fine in moderation. Should teenagers smoke it? Probably not, but he came home, he was fine, no harm done.