Imo
Discipline builds discipline. Drills of any sort is very difficult and not for everyone, there is no set drill or thing that builds this in an individual.
If your (that persons) goal is to become better at their sport or better at a skill (problem within that sport) then they need to work on it. Understand that it will be boring in the beginning but get easier or "more tolerable" as time goes by. A C player needs to work on the basics, so like someone else mentioned, shooting stop shots is about as basic and early foundation for building discipline.
This person needs to understand that it's a "marathon" and not a "sprint", no one drill only teaches one thing. Use each drill to work on form, fundaments, stroke (all aspects of it), technique, delivery and timing, picking any combo of things to work on the most over the rest.
i.e.
while shooting the stop shot let form be your number one goal in the beginning, with timing next and cue tip placement to follow, caring less about the actual "stopping of the ball". Once form and XYZ builds it will become a part of you and thus "cue tip placement" to fall into place, cause you will learn that cue tip placement changes depending on the shot and situation and you will never play a single game if you think about working on all of the options in pool before applying them. (Fundament/Form is the number one breakdown of any pool player and yet the least thing practiced)
This is all just advice (i'm no pro and this is just IMO) so do whatever the ______ you wanna do
(apologize for the last comment, it's just a catch phrase that someone i follow use and i find funny "but true")