I suggest that the choice to take weed may have changed her attitude as lung cancer is often about wanting to quit or get out of life. I have seen cases like these clear without weed or anything else.
A second cause of lung cancer is about fear, a strong shock perhaps? Weed or the choice to take courage to use it could possibly mellow that out too.
But weed in and of itself, does not cure cancer. Plenty of studies show that weed is actually more carcinogenic that tobacco.