I have been in this situation. I did not tell.
I knew the mom very well - and was observing it myself and realized, she just chose to ignore it. Some parents do. It was my teen's body language (uncomfortable) that alerted her to how awkward it had become for my child. She brought that up to me.
Then I just said "Yup" and that's when she realized how bad it was. It ruined our kids' friendship. I didn't add to it. She blamed another child for what had happened, and that was how she kind of handled it.
So there likely won't be much point in you bringing it up. That same child is now involved with a lot worse stuff ... some parents just don't want to know, or don't want to handle it. Not saying that's why all kids get into trouble, but some parents do turn a blind eye.
I have other friends (whose kids are my kids' friends) who say 'tell me if you hear anything' and we have a pact. We talk about what our kids are up to. We are each others' eyes and ears. The kids' (teens) know this. Our teens talk to each other. Some of the dads are coaches, etc. It's kind of a nice group of parents.
It really depends. Some parents look out for each others' kids and the teens know this.
But if you don't know them very well (like close friends) and don't have that level of trust, etc. I would not personally.