Saw my best friend smoking weed, would should I do?

My best friend and I promised to each other that were not going to smoke weed. One of our mutual friends, Bryan, started to smoke but we ignored it because he wasn't close to us friend wise. But one day(long story short), I went to my best friends house and peeked through the fence and saw them smoking in their backyard. I'm angry that he broke our promise, but at the same time im angry that Bryan pressured him to smoking. I dont know if I should tell them I saw them smoking or stay quiet. But I have to do something. I dont want my best friend to turn into some addict or get in trouble because someone pressured him. I regret not jumping over the fence doing something....please help, ty