Dude, Jack is living the life in Otherville. He's got new friends, new clothes, new outdoor activities. He and Tom probably fix cars in their spare time.
It doesn't matter whether Jack is pretending or not. The simple fact that he made any kind of agreement with the Others on behalf of himself isn't going to sit well with Kate. Not after all she's been through on Alcatraz with Sawyer. They could've very well been dead if Pickett had gotten his way. Kate hasn't forgotten any of that. If Jack could make secret pacts with these people, what else can't he do? This revelation will have a drastic change on their relationship. And I reckon not for the better.