The problem with these "drug offenders" is that most of them commit other crimes (burglary, robbery, identity theft, etc.) to support their habits. That may not be the case with this girl's marijuana use, but the drug business is a dangerous game.
I have my own stable of snitches. When I sign a new one up they always ask two questions.
Q: "What am I getting out of this?" A: "Not being ass-raped in prison."
Q: "What's the downside?" A: "The guy you're snitching on could kill you."
I make it perfectly clear that there is danger involved in their service. I also make it clear that although I will protect their identity, the bad guy may still figure it out. The CI actually signs a contract stating that he/she is aware that neither I nor the department are responsible for their safety.