In the original story, Maudgalyāyana cannot assist his mother with his own magical powers; he can only do so by the power of the dharma. The fact that she is saved when he offers a meal to the monastic assembly reflects the widely held idea that the transfer of merit to the deceased is most efficacious when ritually mediated by monastics, especially those earnest in practice and pure in their vows. … Nichiren presents an alternative explanation, showing how he adapted traditional Buddhist stories to his Lotus exclusivism:

The Urabon service began with the Venerable Maudgalyāyana’s attempts to save his mother, Shōdai-nyo, who on account of her miserliness and greed had fallen for five hundred lifetimes into the realm of hungry ghosts. But he could not make her become a buddha. The reason was because he himself did not yet practice the Lotus Sūtra, and so he could not lead even his own mother to buddhahood. But at the eight-year assembly on Vulture Peak, he embraced the Lotus Sūtra, chanted Namu Myōhō-renge-kyō, and became the Buddha Tamālapattracandanagandha [Tamālapattra Sandalwood Fragrance]. At that time, his mother became a Buddha too.

You also asked about offerings for hungry ghosts. The third fascile of the Lotus Sūtra says, ‘It is as though someone coming from a country suffering from famine were suddenly to find a great king’s feast spread before him.’ … When you make offerings for hungry ghosts, you should recite this passage from the sūtra and also chant Namu Myōhō-renge-kyō.

Claims that one person’s religious attainment would simultaneously benefit that individual’s family members, sometimes for seven generations in each direction, was common in Nichiren’s time. They express a confidence, grounded in Mahāyāna notions of interconnection, that one’s own practice affects others across time, space, and the boundaries of life and death. Nichiren here assimilates such ideas to the practice of chanting the daimoku, the title of the Lotus Sūtra.