It has to be one of the highest honor to be the mother of the Buddha and so her merits must've been boundless. After she's reborn into the Tavatimsa Heaven, the Buddha visited her and taught Her the Dhamma. It's just impossible to see anyone who has such caliber, born into such favorable condition, under the instruction of such a supreme teacher, to fall back and retrogress. If she has not attained the highest fruit, there's no doubt she will. There's only 1 direction for Her now: Forward.