Biological Children of Fostering Parents Taking Drugs

Hi, a person I know has both foster and biological children living with them. The biological child is a teenager and on rare occasions smokes cannabis with their friends. It is never done in the house and it is never done around the foster child. What is the general procedure that the social workers and the law follow in terms of an instance such as this? If they got caught by the police when they were out and about would that have any knock-on effect that might cause the foster child to be removed from the parents' care? I am not looking for responses that are from the perspective of a moral high-ground; I simply want to know what the law is surrounding that. Any directions to other source material would be great! Thank you.