and IMO one of the hardest things to reconcile her motivations and behavior with in the book is what comes off as selfishness in her career and her affair with Ty vs. making any kind of effort for her husband and children. Here, it is much easier to imagine that a) she is feeling a deep kinship and attraction to Ty while b) she happens to also be growing apart from her husband.
I understand that it's hard for parents to balance work and family, and even more so with a special needs child. But in the book, Tina came off as SO UNCARING in regards to her family and the fact that her husband was doing all the parenting because she was never home. And part of that was due to her, selfishly, spending time with Ty....to escape her family life. So, yeah, glad they (seemingly) jettisoned the kids and are presumably just going with two people who have drifted apart.