According to Nichiren, no mentor, master, nor living teacher necessary

"This is what the Lotus Sutra means when it says, “This is what is called observing the precepts.” The Reverend Annen in his On the Universally Bestowed Bodhisattva Precepts states: “The Lotus Sutra is saying that those who are capable of preaching the Lotus Sutra deserve to be called observers of the precepts.” That is, such persons do not, as described in the sutras prior to the Lotus Sutra, have to observe the precepts under the supervision of a teacher. They have only to put their faith in this sutra to become observers of the precepts." -- Nichiren