It isn't clear whether "a brother who through his own fault leaves the monastery" is a brother who has just been expelled by the Abbot a la Chapter 28, or whether St. Benedict is changing the subject to address all members who may leave, by their own will or not. In any case, it is worth noting that our Father Benedict makes no stipulation that would disqualify those who have been expelled through the process laid out in Chapter 28 from re-entry here in Chapter 29.
After all the time and effort and pain that the community has spent to bring the defiant back to the path of their vows, and after the best efforts of the most skilled "physicians" have failed to heal, one would think that the Rule would say, "That's it. We wash our hands of you." But it doesn't. It says that this process may happen, that the community must be willing to bear the painful burden of failed corrective measures and expulsion, up to three times for an individual.
Think of an employee in an organization whose recalcitrant behavior forces her boss to take her through a carefully conceived, skillfully implemented, and very costly series of disciplinary measures intended to bring the employee back to good standing. But her recalcitrance remains firmly fixed, and she is ultimately fired. If that former employee were to come back a year later claiming to have learned her lesson, who would expect that organization to offer her a job? And to do it three times?
This is a lavish mercy. This is true sacrifice.