We know

  Patty was a happily married woman.  She liked the man she married, and she liked his friends.  She thought she could trust them, but she was wrong.
  Dennis was single when Patty met him.  Her husband Frank worked for the same hospital as Dennis.  The couple started inviting Dennis over to the house to play cards when they needed another person.  He got invited to bar-b-ques.  He was even invited to holiday gatherings.  They kind of sorry for him, because he didn't have any family near by.
  Dennis loved the attention, he felt like everyone should be focused on him.  So eventually, when he wasn't the center of attention, he began to lie.
  The lies were small at first.  He would claim to have gone important places and done important things.  No one really believed him because they knew he had really been at home watching  TV.  They thought the lies would go away if they didn't pay them any attention.  They were wrong.
  Dennis figured out that lies about people and places were only interesting if everyone know the people and places.  That is when things went wrong.  Dennis started telling people that he was sleeping with Patty.  He was dating a nice, but plain woman named Diane at the time.  He told people that Frank didn't care if his wife was sleeping with soneone else.
  It took a while for this rumor to get back to Patty and Frank.  By the time it did, Dennis was engaged to Diane. 
  Patty and Frank were angry with Dennis, but they figured the best way to get even was to beat him at his own game.  So they played it quiet, they pretended to not know about the lies.
  Patty and Frank went to the bridal showers, and the bachelor party. They went to the rehearsal dinner and the wedding.  At the reception they toasted the bride and groom.  They were even asked to house sit for the honeymoon.
  Patty and Frank got two dozen rolls of toilet paper and "decorated" Dennis' house.  They asked a pregnant friend to take a few tests.  All the tests showed positive!  Patty and Frank wrote all over the windows in shoe polish.
  When Diane came home with her new husband, he had to explain why the windows said, "Patty is pregnant, we know who the father is!  Happy Father's Day! "  Dennis also had to explain the positive pregnancy tests in the mail box. 
  Diane got a divorce, allimony, and half of Dennis' stuff.  Denis got a very good reason not to lie.  Patty and Frank got even. (Maybe ahead!)