Final answer:
Judkins left the crew in Part 2 "On the Trail" of Lonesome Dove after a masked man warned him against riding for Jane Withersteen, but he remains loyal to her and does not intend to abandon his duties unless dismissed.
Step-by-step explanation:
In Part 2 "On the Trail" of Lonesome Dove, the individual who left the crew is Judkins. He made this decision following a threatening encounter at his mother's house. A masked man warned him against riding for Jane Withersteen, and the encounter left Judkins feeling the need to pack guns for protection and remain vigilant. Nevertheless, Judkins expressed a strong sense of loyalty to Jane and declared his intention to continue riding for her unless she dismissed him. This is indicative of the harsh realities faced by the characters in the story, dealing with threats, loyalty, and a rough landscape.