Can Lithum Be Substituted With Weed To Cure Bipolar?

My husband takes 1200 mg of lithum and for bipolar he smokes weed every day several times a day he says this makes him not have to take his lithum for bipolar. He gets in these manic moods and I can deal with it anymore. Now if a doctor can tell me that weed cures bipolar I will eat my words. So please if this is true or not