According to Jim Recap: The Helmet
When Cheryl tries to buy Jim a signed football helmet for his birthday, she finds that someone else is bidding on it too. Before she knows it, she's getting into a discussion with this other bidder in an attempt to convince them that she deserves it more than they do. During their conversation, she finds out that they buy sports stuff and hide it from their wife all the time. Then she finds out that she's the wife that has no idea!
So she uses her newfound position of authority (Jim doesn't yet know that it's her) to convince Jim that maybe he should buy her a new dryer instead of spending money on things that he hides and never uses. Jim actually does this, too.
Then she finds his stash of signed sports balls and pretends that they are making planets one day when he comes home. When he freaks out, she shows him that she just found the balls and was pretending - but she doesn't let on that she is the one that he's been chatting with, saving that for later ammunition.
When Jim does find out that it's her, he's mad - but in the end, it comes out that what he was looking for was a friend, and all along he's had it right there in front of him. And he gets the helmet, too.
