Bah, apocryphal.
fentanyl has generally speaking only made it's way into the supply of non-opioids like cocaine as the result of contamination, not intentional adulteration. When we talk about fentanyl, in reality what's out there is perhaps some true fentanyl but a lot of fentanyl analogues that are much more potent (lethal dose in the mcg range). Often the people moving heroin are also moving cocaine and other drugs, and using shared surfaces to weigh and cut the product for street use. They aren't properly cleaning surfaces and instruments and thus fentanyl is ending up in all sorts of powdered drugs
fentanyl on weed has probably happened in some instances of intentional poisoning of an individual but I'd file this under similar in risk to your child being given an edible, or candy with a razor blade on halloween
There’s even been a rash of fentanyl overdoses in nyc from marijuana (as reported by the victims). It’s thought that mass produced weed is being grown with plant growth regulators. These make the plant grow big, dense buds. It also makes the plant produce less thc. It’s believe that the plants are then being dusted with fentanyl to replace the lost thc.