It wouldn't be a concern for me. Why is it "scary"? The US president smoked pot, Canada is about to make it legal and it already is in many countries, etc. Teenagers will all be exposed to pot and socially pressured into trying it. I know I have. Then I moved on. You can't prevent it by stopping GF.
By the way, your conversation will start with "Hey, I heard from the neighbor who's babysitter goes to school with D17 who told her..." It will not sound very convincing, nor hard to dismiss.
You'll need to have the drug conversation with your D13 sooner or later, regardless of whether you will be hardline or not, and this might be an opportunity. Or you can wait. But you can't stop GF from doing anything by speaking to no-good Mr Fantastic. It's just another opportunity to show him how powerless you are.
M39 D6 D3 (at S) S 2014-09 D 2016-09
"You can't start a fire sitting around, crying over a broken heart" - Bruce Springsteen.