To tell her or to not...

So me and my boyfriend stayed the night over my best friends and her bfs house. We decided to stay a second night, but she had to go to work. I decided to just stay and I'd just chill with the boys til she got home.. So, of course they got into "guy talk" and he mentioned hes been smoking weed (she highly dissaproves) I was brought into the conversation about my opinion.. I tried to stay neutral about it ( I dont mind weed) I never told her because she was pregnant and I didn't want to worry her by it... And I never seen him smoke, he just told us he did.

So, they had the baby now and they've been really getting into it bad. He keeps losing jobs and she's stuck paying for everything. I decided to give them my car (they would pay later) so I drive him to the bmv to get it registered and whatnot and I was trying to motivate him about getting a job and where he'd wanna stay. Basically, I brought up me going to a temp service and all I had to do was drug test and was telling him how easy it was. He said he wouldn't pass one.... So I gave him a huge lecture on it and why I thought we was wrong for that. He's putting her in this position she doesn't have to be in just because of that...

My dilemma is that I still haven't told her he told me at all... I just don't know how to say it and then say he told me almost a year ago and I never said anything... I feel so bad, but I don't know how to tell her..........