Suggesting that study participants use a drug of unknown purity, composition, and dosing would definitely not be ok or safe. Encouraging illegal activity generally is not ok. Interviewing self-professed users without instructing them to use the drug would likely be ok but of limited value.
Parent did not say they would be purchasing in a state where recreational dispensaries exist or where weed is legal at a state level. Those are unstated assumptions.
I don't know about other legal states but here in Washington the purity, composition, and dose of cannabis products are highly regulated and come with standardized labels.
