There are some assumptions here. The child might not have eaten her mother's cannabis edibles but got them in another way. Also, are they cannabis edibles or CBD edibles, a very different thing but often described similarly. This mother may be a good Mum but desperate.
As to "gentle parenting", you would soon find yourself in trouble with Social Services if there was a good hiding being meted out. My grandchild throws things, hits out and does the most dreadful things and SS are involved because her parents asked for help. Advice is dependent on the Social Worker involved but almost without exception you are not allowed to punish by removing items or often a reward if they do what they need to do and then don't give it to them if they don't. You must never, ever say that you wonder if the child is Neuro Diverse (ND) either because then you are blaming the child for their behaviour. It MUST be something the parent is doing. They can tell you very definitely what you can't do but have no advice as to what you can do. I kid you not but even the school stepped in and suggested that Social Services were making things worse, not better. After 8 years with on and off "help" they are at last considering that maybe she is ND but refuse to agree with school as to what form it takes. Meanwhile the violence escalates and as she heads towards teenage years, other problems are gaining traction. There are times when her mother would welcome a good tasering (joke)!
I am not saying I never, ever smacked my children but there were limited occasions and only when they insisted on sticking their fingers in the plug sockets and I considered it safer to smack than let them electrocute themselves. Five of the Six knew the rules and knew that they would face my disappointment if they broke them and, on the whole, that was enough. The last knew the rules and didn't break them but would make life as difficult as possible in every way.