arents have bsolute say in who their minor kid sees. If parents find out they would be within their rights to forbid grandma from seeing granddaughter.Lw will lose her granddaughter's trust if she goes to the parents. The best way she can help Grace is to keep the parents out of it.
There are ways to finesse this. Grandma needs to help kid on a tangible program of change and then when positive things are happening, she should encourage kid to share with parents.
Meanwhile, since Grandma knows who is supplying weed, Grandma can address that making sure supplier won't plant some stuff metaphorically at Grandma's house.
Grandma can spend a lot more time with kid. Tutor or help with homework, hang out at Grandma's, go to movies, go to mall,offer to drive the kid somewhere and then hang around, anything that distracts the kid and provides a viable alternative to booze and dope.