Who Do You Believe?

A very interesting case was presented to me a couple of weeks ago. It's something I am still unsure of as to how to answer. Before anything, of course, let me give some background.

This story is about 2 very good friends of mine- Kay and Jason.

History: Kay and Jason have been married for almost 5 years. They were together for 3 before that. They have 1 child, a daughter, who is 3. Roundabout the end of the 3rd year of their marriage, they went through what is commonly called a "rocky patch". Jason had an affair, and Kay found out about it. They were able to get through it though. It wasn't easy. I watched from the sidelines as Kay struggled to find trust and Jason struggled to find redemption. Now they appear ok. Until Kay called me with a dilemma.

Background: In the middle of the whole bruhaha that was the affair, Kay had discovered that one of Jason's close friends knew about the affair from the beginning. Why would this be a big deal? Well, this friend had also become her friend, or so she thought. Thus, the discovery of the knowledge felt to Kay like a betrayal as well. Needless to say, she deleted all info of the friend (whom we shall call Sandra) from her contacts- cel phone, email, multiply, friendster, facebook.

Present: Years had gone by and suddenly Sandra had found a way to contact her. It seems one of Kay's officemates knew Sandra and it was through him that Sandra got Kay's contact details. Kay told me Sandra contacted her telling her she wanted to meet to talk. Kay considered and decided to hear her out (over my strenuous objections). It was this talk that was interesting.
Sandra presented all the arguments for her case, for why she did what she did (meaning: keeping the knowledge of the affair a secret for a long time, sometimes even asking normal couple questions about it, regardless of the fact that Kay was also supposed to be her friend) of friends being caught in the middle, having bad personality traits winning in situations of drama, etc. She also said that she understood Kay's ill-feelings towards her but she told Kay she wanted to explain her side and she hoped that, through this Kay would have a more accurate picture of what her position was, and that she could save the friendship. Kay told me that her arguements were quite convincing so, in the spirit of being open-minded, she decided to give Sandra a chance. After this, Kay and Sandra met one more time over coffee just to catch up and try to make a go at mending the friendship.They talked about their lives at present, what was new, what was old, etc. It was during this meeting that Sandra involuntarily divulged to Kay that Jason had mentioned to her very recently that he just wasn't feeling a friendship connection with Kay and that he was disappointed that Kay always wanted to spend her free time with the daughter. Now, I had to struggle to keep from saying "I told you not to talk to her anymore". You should have heard Kay explaining how surprised she was to hear that. She told me she just smiled at Sandra, thanked her for the head's up, and then went home. When she and I talked she explained that she was confused about what Sandra had said because 99% of the time, she and Jason were together, either out with friends or at home relaxing. Kay said she did spend time with the kids as well but this was usually at the same time Jason would be at the office (he works in real estate hence the odd schedule).

The Dilemma: who should Kay believe? Should she go with what she sees and feels, what she knows from the way she interacts with her husband, and from what appears to be a good, mending, growing relationship despite the fact that she had been wrong about their state of affairs before (circa the affair); or does she believe Sandra? Who has brought dependable information to her on several occasions before, who would appear to have no reason to be destructive, but who naturally cannot be trusted the same way anymore?

What do you guys think? I'm stumped.