Of course, she actually left him for Locke, which will be revealed in the exciting second season finale, along with the fact that Jack was high on Charlie's heroin when he botched the surgery on Locke's gall-bladder and paralyzed the man, all of which could have been reversed with a certain surgical tool (invented by Sayid) shaped like a plastic airplane, which, unfortunately, had been stolen during the surgery by Kate while Jack and the nursing staff were distracted by a passing derelict (who will be later revealed to be one of the Others who kidnapped Walt). It will turn out that Jack's ex (who is also Locke's ex) is living beneath the bunker with Sayid's girlfriend and a bunch of robots they made out of spare computer parts, and all of them sit around cracking wise while watching episodes from the first season.
Mark my words.