I thought it might be worth noting that when Ennis tells Jack about Cassie, his relationship to her could effectively have been over by then. Jack and Ennis's final meeting probably takes place in the May of 1983 and his final meeting with Cassie is probably a month or two later, say in the June or July of the same year. At that meeting, two things emerge. The first is that for some while she has been trying to contact Ennis for she says to him:
I left word for you with Steve at the ranch. And you must have got those notes I left at your place.
How long this has been going on, we don't know. But it sounds to me as though Cassie's attempt to contact Ennis might well have taken place over a few months. The second thing to emerge is that at some point in this process, Cassie has decided that her affair with Ennis is over and she has had time to meet and establish a relationship with a new boyfriend, Carl. How long this has been going on, we don't know, but when we see Carl, it becomes clear this isn't their first date as Cassie says of him:
Carl? Yeah, Carl's nice. He even talks.
Put these two things together and overall we may be looking at a period of, say , four to six months and so, when Ennis tells Jack about Cassie, Ennis may be describing something which effectively belongs to his past rather than the present. However, whether or not Ennis is prepared to fully acknowledge this when talking to Jack is far from clear.
But obviously he has to acknowledge it when he encounters Cassie with her new boyfriend at some point during the summer of 1983. This means, that, without Jack, his isolation is even more complete. This, along with Jack's threat of "quitting" him, may have caused Ennis to start rethinking his life during the late summer and early fall of that year. It is Ennis, of course, not Jack, who initiates the November meeting at Pine Creek and maybe, at that meeting, Ennis was thinking of offering Jack more of that "sweet life" than he ever had been prepared to offer in the past. Anyway, I like to think so.