Or maybe she was looking for companionship. Maybe she was lonely. Even if she didn't know about the affair, she probably sensed something. They say the wife is always the last to know, but they probaby usually suspect something and choose to keep their suspicions to themselves for a variety of reasons. And her "best friend" probably wasn't all that available to her (at least in any genuine, caring sort of way). Also, maybe she was the type who didn't have a lot of friends she could turn to. I'm the same way - most of my social activity is online, because I'm very introverted and prefer to keep to myself and have my own time and space. And with Jennifer being a SAHM, I'm sure she didn't get out much, being busy with the kids.
Being in a troubled relationship can be an extremely LONELY place. So as her marriage deteriorated, she probably got lonlier and lonlier, increasing her desire to connect with people online.
JMO.